They get no extra money based on what? If you were asked to do your bosses job for 3 months would you do it without extra pay?
uh, this happens all the time in business... All the companies I've worked for do this (and I have done this multiple times with some of my employees). It's basically seeing how someone performs with more responsibility. Gives you an idea if they are an option long term.
As far as our assistant coaches go: Yes, maybe, maybe they have something like that built in their contracts, but I think that's reaching a bit. And even if it was, considering our assistant coaches resumes, I doubt it would be that much more financially honestly.