Somebody would watch it, my girlfriend watches a reality show that sets people up on first dates, all in the same restaurant. She also watches a show about the people that the CBSA deal with, I guess it's like a polite version of Cops.
But that's kinda the thing, I don't personally see live rehearsal taking off, but as others including yourself I believe have stated, practicing some songs off camera, and playing a sort of setlist, maybe with some banter in between, could do well, but that would probably best be in between a regular schedule of uploaded content with a channel that's got an established draw and can entertain people. This primary revenue, as far as I can tell, is based mostly on people looking at ads. A longer video could get people looking at more ads, but only if it draws people in and can keep them viewing longer. It'd be easier to draw people in to a longer thing after people decide they like looking at your shorter stuff. At any rate: I'm fairly dumb at this and really can't back up what I'm saying, this is all the conclusions I've come to from looking at stuff on youtube casually - this advice is offered freely because I couldn't ethically accept money for giving it.
Edit: also, your idea has potential, fake fighting, dramatic depiction of the songwriting process, physically attractive women (and I would assume men as well) . Look at the food Network, sure, commercial kitchens are stressful environments, but most of that stuff is just overdramatic, hypercompetitive crap anyways. As much as I respect the skill of those involved, I can't watch that stuff.