Deleting images in camera is frowned upon because you can easily make the wrong judgement about an image that may have a future historical use, despite its less-than-stellar characteristics. It is also a bad idea if your editing and/or post-processing software requires continuous numbering with no breaks. It's a bad idea if you work for a portrait company and they tell you not to delete... They want to evaluate your efficiency in capturing excellent poses! It's a bad idea if you are doing forensic or legal or investigative photography, because someone will suspect you're hiding evidence.

Deleting as a physical act usually does not harm the card or other images on the card. There were some isolated issues with this a decade and longer ago, but those issues are no longer relevant. That said, I NEVER delete images from a card until I've got the good ones in three other locations. At that point, I'll simply format the card — in the camera.

I NEVER format my cards in my iMac, because formatting cards in Lumix cameras puts a special set of files and folders on the card that are required by my camera. (When I buy a new card, I format it in the camera immediately, so I know it is ready for use.)

I NEVER import images into Lightroom directly from a card. I always want a working set and two copies before I reformat the card. So... I copy my events from SD card to folders on my main drive. Then I import them into Lightroom, culling, rating, and deleting a few from disk. (I only delete images that are so bad, I know I'll NEVER use them.)

Another reason I do this is that I keep all my images in dated folders with plain English event names on them. My mind is very date and event oriented, so it works for me. It's important, because I use several other applications to process images, and they are not all compatible with Lightroom's "Edit In..." command.

Before processing, I'll back up the raw files to another drive and either to the cloud or a DVD. JPEGs created in camera are culled in Lightroom, then edited and saved with the raw files, if I have raw files. (There are some critical types of work where raw files are simply not helpful... There are other critical types of work where JPEG files are a complete waste of space and time.) JPEGs created in Lightroom from raw files are for specific external uses, so they get filed separately or erased after use.

I have for the first time heard that one should not delete images from the card while it is in the camera. Because it corrupts the card? I have been doing this daily on long trips to weed out bad shots and to avoid being overwhelmed at the end of the trip. Also, when you use Lightroom, you are supposed to download images via Lightroom and Lightroom is not an easy way to deal with a huge number of new images. Anyway, I will download and then reformat the card in the camera, as suggested, if that is truly the right way to go.

I have for the first time heard that one should not delete images from the card while it is in the camera. Because it corrupts the card? I have been doing this daily on long trips to weed out bad shots and to avoid being overwhelmed at the end of the trip. Also, when you use Lightroom, you are supposed to download images via Lightroom and Lightroom is not an easy way to deal with a huge number of new images. Anyway, I will download and then reformat the card in the camera, as suggested, if that is truly the right way to go.

I must have deleted thousands of images directly in camera while testing and chimping over the years, involving dozens of cards from 1Gig to 256 "super speed" cards. I've done it on 5 MP point and shoots and on 24 MP DSLRs. I have never, ever had a card corruption problem on any of those cards.

I do strongly advise reformatting the card in camera when you do so, but other than that aspect, I think most of this is an old wive's tale. I'm sure willing to stand corrected if someone can come up with a reasonable explanation as to why in-camera deletion is more likely to corrupt a card than uploading and deleting.

Andy

PS: If it turns out there is really a good reason for doing this, you still don't have to pass everything through LightRoom, including images you're going to delete anyway. I often upload from a card reader to a "Temp Upload" folder on my desktop and then delete all the test shots and junk before importing and copying via LightRoom.

I do think that generally reformatting the card every time you put a card back in the camera is a bit over-the-top.
The in camera formatting just does exactly what the file system does on your computer. It writes a known file structure onto the card (ie directories where the camera can find them and with the right names). The camera uses these directories in the same way a PC does - that is it writes files to them.
If you delete images in camera this removes the directory entries from the card - same as a computer.
Probably the reason why some people need to reformat the cards in the camera (and I maintain that you don't need to) is because they don't know enough to be able recreate the same same directories as the camera requires. But that begs the question why would you have to reformat the card anyway (in camera or PC) if the direrectories haven't even been changed.
I cut and paste from the cards and then reuse them. No formatting - other than I know to format a new card in the camera to make sure the directory structure is correct. I have been doing it this way for about 5 years and not a single issue.
I am starting to think that reformatting is either a myth or something that started back 20 years ago when there maybe were some issues.

I'd have to see some convincing evidence to show that deleting images in-camera is a bad practice. The file system on the memory card is for all intents and purposes the same as the file system in our computers, and we know that computers can handle deleting files all day long with no problem. That said, it could be that the camera's file system might have potential problems handling fragmented storage on the card...where files might get split into pieces due to the "holes" in data resulting from deleting files. Or it is possible that when those holes are left after deleting the image files, the camera operating system just doesn't go back and use that space again until the card is reformatted.

I've become pretty picky around how I handle and use my memory cards, but this is not something that I've ever considered a no-no. It's not something that I'm aware has ever created a problem for me or anyone else over the past 13 years or so.

Bottom line...I'd be very interested to see some specific, credible information about the what, why, and how of this proposed danger.

I always delete images that I know are failures using the camera controls when I have the time and the opportunity, and I have been doing so for almost fourteen years now. As soon as practical, I remove the card from my camera, connect it through a reader (internal or external), download/copy the files to my computer hard drive (using Photoshop Elements), and allow the PSE Organizer downloader to verify the copying to the computer hard drive and delete the files from my card (I do copy the new download folders to two additional hard drives, too). I follow the same procedure when I am traveling, but I use a pair of portable external hard drives instead of my laptop hard drive (and then copy to my primary data drive when I return home). I don't think I have ever formatted my cards in camera, except perhaps when I first purchased the cards; I believe I usually insert them, and use them if they work, and only format if they do not. It may only be luck, but I have never lost a picture or had a card failure. I shoot in RAW + .jpg (large), and I use mainly large cards (32- and 64-Gb SD and CF), saving to one card, with autoswitching to move to the other when the first is full (only rarely do I fill up a card before I get a chance to download). I do tend to leave the same cards in the camera all the time--so I really only use one SD card (64-Gb) and one CF (32-Gb).

I have some 77,000 image files in my photo directory (probably more like 38,500 pair of RAW and .jpg).

I have for the first time heard that one should not delete images from the card while it is in the camera. Because it corrupts the card? I have been doing this daily on long trips to weed out bad shots and to avoid being overwhelmed at the end of the trip. Also, when you use Lightroom, you are supposed to download images via Lightroom and Lightroom is not an easy way to deal with a huge number of new images. Anyway, I will download and then reformat the card in the camera, as suggested, if that is truly the right way to go.