Versatility for a prepared caster depends greatly on a campaign. There are only so many spells that can be useful for a given situation. What happens if we need to fly two times in a day? Game over? That seems kind of dumb.
What about if you have two or three similar combats, but now I have to develop completely different (and possibly ineffective) strategies for each one, while a martial adept or non-prepared caster can use the same system?
It just makes life harder on the player and over-all less fun, in my opinion. As was said before, leave the ToB rules with ToB.
Try talking to your players, too. See what they think. If you want them to play to their versatility, challenge them so that having prepared different spells will be helpful.
In the end, this whole idea sounds like more work for the players and DM and less fun.