Didn't someone on the Bioshock team come to Superbot and say they'd like to see Big Daddy in the game?
I think they went to Sony, actually.
During a panel at SDCC, one of the people asked about certain characters and Chan Park/Omar gave a really detailed answer about how little control they really had. One of the things that came out was that there were certain characters Sony actually told them to put in.
I've always felt that BD was one of these characters, who was put into the game as a way to maintain the good relationship going between Irrational games and Sony right now.
Ultimately I don't mind, he's pretty fun and I love Bioshock, but it's pretty illuminating all the same.
If I haven't mentioned Trivio in my post, it's because she is so beautiful and precarious that I just wanted to show her how much she means to me.