Well, for those first fifteen hours, you really don't need three-person parties. The encounters are designed for the parties you have. Final Fantasy IV offers five-person parties, and yet you don't even get a full party until Yang joins hours into the game (only for the party to be completely broken up within another hour or so after that).
Eh, I just didn't like that approach. For whatever reason, if a game is meant to let you have 3 party characters I want to have 3 party characters as soon as possible. Maybe if a game was able to have 5 party characters but limited you to 3 or 4 it wouldn't bother me so much, since 3 still allows for a good range of tactics and some variety, but 2 just strikes me as being a bit too stunted.
As for the paradigm roles and character building, different people are bound to play the game in different ways. Once I had my full party available, I spent the vast majority of the game rolling with a team of Lightning/Fang/Hope, and developed the other characters on the side. I never even came close to maxing the crystarium completely out for any of the characters; I just played the game at my own pace and moved on from Pulse when I felt that I was ready for the rest of it. Other people would be more inclined to max out every character and would be happy with a party compose of jacks-of-all-trades, or who would try goofy things like trying to make Vanille the party's best sentinel.
Again it's a personal issue, but I just prefer that in the end, if you level them all up to the max, that the characters still wind up unique in some way. Even if I never get anywhere close to the theoretical point where they are maxed out in everything, the idea that I could get to that point bugs me a bit. It also doesn't really encourage me to play deep into that game. That's just a weird peccadillo I have regarding rpgs.
I guess both these issues come down to making me feel limited in my options. For the first 20 hours of the game I couldn't choose my own party, I didn't have much in the way of choice regarding how to build my characters, and I only had one path to go. It was limiting and I didn't appreciate it for those reasons. I did appreciate it for others, but they weren't strong enough to keep me playing all the way to the end.