I agree with you that a cruise should consist of doing what you feel like doing whenever. That is part of the whole idea of cruising.
But there are also people who love to pre-plan everything and also want to make sure they don't miss certain things. For them this is a convenience.
I don't think anyone really knows yet how much this will be a "necessity" for Oasis. I am hoping it is not one, but if it is then you have to give Royal Caribbean credit for being ready for it. They have this entire system designed and operating before the ship is even launched.
They might find that almost everyone thinks like you (us) - and do not want to preplan anything. That will work beautifully if they show the shows frequently enough. I really hope it works out that way.
What will probably happen is that most people will pre-book things at first. The determining factor will be how well people who DON'T pre-book can still find seats. If they can accomodate people easily without reservations then they can make the reservation system redundant and people won't use it so much. That should be their goal.
It will be a real test of the staff onboard. If they just fall back on the reservation system and force people to use it then its too bad.