I just beat every last single enemy you can beat in the game. Now I only have to redo some mission for a perfect ranking and level up my equipment for total completion.
The farther you get in the game the more control you have. There will be new things added to the battle system for a pretty long time.
I just checked my save files and at the beginning of chapter 9 I couldn't select the battle members while I could in chapter 10. So, it either happens somewhere in chapter 9 or at the beginning of chapter 10.
Looking forward to: FF Versus XIII / LBP2 / The Last Guardian / Yakuza 4 / ICO Collection