Neymar seems to be a very humble guy. He know that Messi is a much better player, in fact he always says so, for example in the club world cup in 2011 when they faced each other or when Pele said that he was going to be better than Messi. He recognized Messi is miles ahead everyone else and that he doesn't come close to him. He doesn't have the Cristiano/Ballotlli attitude.
I think he would be a great addition to the team. Alexis and Pedro haven't been very convincing lately IMO.
The thing is that being a forward at Barcelona means playing in Messi's shadow. The other two forwards are basically there to do a lot of work off the ball and to compliment Messi as best as possible. That's why Pedro players every match despite not scoring, he is an ideal foil for Messi.
I doubt Neymar would be pleased with having a secondary role in the team in Messi's shadow or that he'd even want that - presumably he wants to be the star somewhere in a team built around it, which will never happen at Barcelona.