Why it's true that the game forces certain characters into your party for story-advancing battles, you get complete freedom to pick and choose units for random battles. Personally I didn't find a five-party system restrictive. It's larger than many parties in the numbered FF's.
The unique characters are Ramza, Mustadio, Agrias, Rafa, Malak, Beowulf, Reis, Cloud, Worker 8, Orlandu, Meliadoul, and Byblos. That's 12, unless you count Boco the chocobo as unique (which I don't). Since there's 16 slots total it's actually possible to recruit every unique character in the game and still have space left over.
What does have to suffer if you want to keep all the unique characters is the generic human and monster units. You start out with four humans, and gain three more in chapter 2. And inviting monsters is even worse, since they breed. So my strategy is to breed and poach monsters and fufill propositions as early as possible and then ditch most of the generics once my roster fills up.
You're certainly entitled to that opinion. But I disagree.