I never had an issue getting hours if I wanted them. There were always hours available on the shift exchange.
I actually liked driving the Nova. I didn't like the RTS mainly because the window arrangement for the driver got hot fast and the AC couldn't keep up. I didn't mind the Gilligs but just liked the Nova better.
The 5 new Nova's that they got in late 2010 (these were part of the settlement between WDW and Novabus for bus fires) were actually very nice.
Somehow, I don't think requiring that someone stay x amount of time will make better drivers. IMHO, you either want to work there or you don't. You might find out quickly that guest interaction is not your thing and to require someone to commit to x period of time will make for just a bad overall experience for all concerned.
If you want to be a cast member and enjoy the job, you will do your best. If you don't, you won't. I thought then and still do that it was one of the best jobs I ever had. If things were still the same there, I would go back in a heartbeat (if money was no object), but sadly, it's not the same. It still might be a good job and if I was retired, I might see whats up with it now but MIM's spoiled the fun I used to have with the guests.