Why on Earth are people saying that the game would have taken years longer to develop if it had 6 party memebers? That makes no sense. .
You're right. It doesn't. Even the argument that it would make battles "longer and more tedious" is mostly baseless bullshit. In particular in a game where the initiative queue is mixed and if anything having more characters under your control would mean that you as a player can "inject" more often in the mid of lsequences of NPC moves. A lot of people just love to make up this sort of stuff, for some reason.