Well, I love I Want You, it's probably my favorite Beatles song (along with Day In The Life) and I agree that it suits the Rooftop atmosphere perfectly! (The Beatles should have played it)
Still, I think they should have made the whole thing as realistic as possible. They could have choosen the other songs they played on the rooftop instead.
But I would have been pissed if I Want You wasnt in the game right from the start. xD
Also, @gamegyro56: Is there a video proof of that somewhere? (I didnt see the Let It Be movie) Not like I would count a short guitar part as "playing I Want You on the rooftop" but I would really like to hear it. ^^