If you use your cards in different cams you'll notice that quite a few cameras only show pictures took with this one camera.
So if you chose "delete all" it will only delete those images it knows, made by the cam itself and you'll end up with a partly filled memory card, not being able to take as much pictures as it should.
Same if you sometimes have other files on your memory cards, which eat up space.
Another thing, if the file system ever got corrupt for any reason, delete all won't fix it, while format will fix logical file system corruption.
I'd always use format. Except maybe if the format is extremely slow, taking several minutes (like it was in the Leica M9, not sure if they fixed it by now).
Of course, in case of format everything is gone which, for me, is what I want, but if you tend to put important data or pictures on a memory card without having a backup somewhere else, might not be what you want (but you would not do that, would you).