I've never heard of having to pay your co-workers, so even if it is a 'job' this doesn't make sense to me. If you're all in it together, then you should get paid the same way. Look for someone else to avoid resentment and conflict.
Depends on the business. If you're a co-owner of a very small business such as a partnership, then your co-workers are getting paid out of your stake in the business. Many small business owners deliberately pay themselves relatively low salaries, to keep as much money invested in the business as possible. After all, if they thought their own business was a bad investment, they'd get out of the business.
If you're not a co-owner, which is typical for most US workers, then like you say, it doesn't make sense to talk in terms of paying your co-workers.
If this band has agreed that their financial model is to defer wages / salaries, and simply split all of the proceeds equally, that's their choice, but then having a different wage structure for one of the players would (in my view) require agreement from the entire band. Maybe it's a good business idea, maybe it's not. We can all talk about taking a band to the "next level," but the next level is not a guaranteed success.
It's not a question of what's fair for the drummer, or what constitutes a "pro," but rather, how does the band want to finance itself? And how does the drummer want to finance himself? Not all business propositions are viable.
For myself, I don't tend to demand rehearsal pay, but the bands that hire me as a freelance rarely rehearse, so it's something I don't really need to worry about. By the same token, bandleaders know that if they call a lot of unnecessary rehearsals for low paying gigs, they will have a harder time finding players, so there's a reasonable amount of discipline built into the system.
