The only reason I'd want 6 companions is if I also get to create my own party. Right now I'm fine with four because I'm not keen on dealing with pre-made companions. While I like games like Dragon Age and Mass Effect, I don't necessarily enjoy all the companion interactions. Overly written companions control the narrative too much.. If I get to create my own party like in IWD then that's more fun for me because it's my own soap opera that I'm creating. I get to create the characters personalities and stories as I play the game. If it's someone elses characters then my brain goes all grumbly whenever they do something out of character for the character I'm trying to play them as.
It would be fun to create your whole party IWD style. With the risk of losing out on companion specific side quests and storylines though