Auto ISO can be very useful. In fact, there was a recent post by a well-known African photographer that had moved a majority of his shooting to auto ISO and provided very convincing reasons. There are of course caveats - you can misuse any of the camera controls and produce poor results. In particular, many bodies with this feature allow you to set an upper limit as to how far auto ISO can go. Secondly, the setting that it chooses is right there in the viewfinder along with the shutter speed and f-stop for the photographer to see. One of the only real limitations, is that many cameras do not allow exposure compensation when in manual shutter and aperture and auto ISO, although the latest Canons do allow this.

Hard to second guess the pro shooting the wedding without knowing the lighting and whether flash was allowed or considered intrusive. At my son’s recent wedding, which was professionally and excellently shot, even in a very well lighted venue ISOs in the 8-12,000 range were necessary to maintain a reasonable shutter speed, even with a fast lens almost wide open. While the pro was shooting Nikon D750s and using the range I described, I was shooting a Canon 5D3 (same as your pro) in the 8-10,000 range with many shots at 12,800, and the results were excellent and certainly much better than if I had dropped the shutter speed to the point that motion blur would have spoiled the shots. Auto ISO is just another tool in your toolbox, and like any tool, must be used correctly. In low light situations, where flash is impractical or isn’t allowed using high ISOs may be a necessary compromise, but I will say that even with a FF, I haven’t been doing any shooting at 25,600!

CT - you didn't post any examples, so based on your analysis of the images, please clarify:

1) Was the blurriness due to subject movement or camera shake, or both?
2) Did you try shooting slower than 1/80 with either lens?
3) You mention the difference between f/1.8 and f/2.8. But, were you still at 1/80? I'd argue that you should have been able to shoot a 24mm lens, on any camera body, at 1/30.

Regarding light gathering aspects of the lens at a given aperture: it is exactly the same regardless of camera body. There are differences in the apparent depth of field related to the crop sensor at various aperture values. This is not due to the amount of light hitting the sensor.

If it really was that dark and using flash was not an option, two other options exist:

a) Depending on the source of the blur, a fast IS-enable lens might have been an option. You'd still get blur in the kids not paying attention to the photographer. But, you should have been able to catch static situations or had the kids stop for a moment and smile before going back into their own world. Canon offers both 24 and 28 f/2.8 IS lenses in an EF mount. Canon also has an EF 35 f/2 IS that would have approximated the 50mm field of view on your 60D.

b) A new crop or full-frame camera would have offered better high ISO results than your 60D. I'd prefer not to take the 60D sensor above ISO-3200. But, it will shoot higher and maybe the results are acceptable vs no images at all in the dim light.

Rick, I'm not completely sure I understand your question. I shoot completely manual about 98% of the time. The foremost consideration is shutter speed. That is based on handheld or tripod. Handheld, the usual rule of thumb. The depth of field is the primary consideration. What do I want as an f-stop. What will that f-stop require as a shutter speed? It then becomes a balance or trade-off as to which is more important. A good steady camera is always the important aspect. So, at that point, if I can't get the depth of field I want, I'll use an appropriate higher ISO. If using a tripod, then factors of wind come into play. The shutter speed is determined by that factor and again balancing the depth of field desired, etc. It's a system engineering exercise.

As for metering, I spot meter on the brightest part of the scene. Then increase the exposure an appropriate amount, within my camera's capabilities, to render that portion of the scene the appropriate brightness in the final image. Most of my images, SOOC, look horribly overexposed. However, they are within the limits of the camera and work quite nicely. As a result of this, I rarely chimp. Here's an example of an ideal exposure SOOC.
http://static.uglyhedgehog.com/upload/2015/10/11/1444601010698-d700_2015091301_012_sooc.jpg
--Bob

I have found that when you "almost" have enough light to not need flash the flash will light the main subject but the natural light will allow the sensor to get a bit of a ghost/blur which gives it the "motion" look. I found that out on hummers, they said use flash-it will freeze all the motion. Well it didn't, the wings were still a total blur, so I went to high speed sync and I got wings with the feathers showing etc and just a hint of blur for the motion look. This was in daylight at about 10'.