The last thing Miami needed to do was bring in Steve Nash. Their strength the last two seasons has been terrific perimeter defense, bringing in Nash negates that right away. Miami also loves to trap all over the court, it's hard to try when you have as poor of defensive player as Nash on the court. Also, their best offense is running everything through LeBron like they started doing at the end of the playoffs. Why bring in another player to take the ball out of the hands of the best player in the league?
Because Nash is a better ball handler. Having the ball in Nash' hands could facilitate a lot of movement. Getting the ball to Lebron and Wade in space would be pretty nice. Also, Nash is a hell of a shooter. I do get your point about defense though. Chalmers isn't really anything to write home about defensively though.