With me, it's usually only a matter of 'is this the best camera in this format/line/style?' And if not, do I need one? Sometimes a lower version model will suffice for what I want to do photographically, but then I get a hankering for a sharper lens, or interchangability, or ease of use.
I used to have something in every format from 16mm, to 4x5. Since the death of some of the old formats now means I don't need to be shooting everything on the earth, it has freed me up to concentrate on the more important models I'd like to own. But then I'll find something in a junk shop, and I just can't resist it. Like the little tlr I found that looks like it takes 828 film. It's a dead ender, but will look neat on the shelf.