lighthouse Loc: No Fixed Abode

The ones who consider it to be a red herring perplex me just as much as those that are confused about it.
It is not a red herring, it is a significant characteristic that we need to be aware of when choosing which equipment to use.
But its not that hard to understand either.
For the life of me I do not know why some people have so much trouble with it or try to pretend it is so complicated.

Desert Gecko Loc: desert southwest, USA

Wow, you got an earful. The simple answer is yes, you got it.

Keep it simple and for now forget terms like "angle of view," or "equivalent." What you see in the fx viewfinder with a 150-600 is what you would see in the dx viewfinder with a 100-400.

If you are comfortable with that concept, then you might look into ramifications, like the effect on depth of field when composing an image identically with one or the other (because distance from the camera will have necessarily changed), or that using an fx lens on a dx body will produce an image much sharper on the sides and especially in the corners because only the sharper, center part of the lens' image is falling on a crop sensor, etc. I'm not trying to complicate anything, but offering a couple things to consider when you are ready that many people overlook. There are differences between the two setups that prevent a true "equivalence."