Yes, Nash is a Hall of Famer, but it makes no basketball sense to bring him to South Beach.
First of all, there is the fact that Miami already has a point guard. While Mario Chalmers is no star, he's exactly what the Heat need out of the position. He hits three-pointers, he plays defense and he doesn't try to dominate the offense.
As cool as Nash-to-LeBron alley-oops would be, they wouldn't get those things out of Nash. Nash needs to have the ball in his hands to be effective. Every possession controlled by Nash is one not controlled by LeBron.
And do you ever really want to take the ball out of LeBron's hands?
While Nash is an excellent three point shooter, he's a complete defensive liability. Miami prides itself on defense. While they'd definitely be able to cover for Nash, it doesn't make sense to risk their best attribute just for a player they don't need.
Miami also has a developmental point guard in Norris Cole. Cole already struggles for minutes in Erik Spoelstra's inconsistent rotation—how is he going to improve if all of his minutes are lost to Nash?
And how would Dwyane Wade adjust to playing with Nash? He's had enough trouble playing second fiddle to LeBron, and while he'd undoubtedly keep that role even with Nash, he'd lose a ton of touches because Nash needs the ball.
Miami doesn't exactly have a ton of money to spend. They have their mini mid-level exception to chase a free agent with and that's it. They need a center (they've been covering for Joel Anthony for way too long) and potentially a replacement for Mike Miller if he chooses to retire.
Spending their limited funds on Nash just isn't a smart basketball decision. All he does is add another star to a team that doesn't need one. It might sell a few more jerseys, but do the Heat really need to do that?
They already have a perfectly capable point guard in Mario Chalmers and a nice young backup in Norris Cole. They're already the champs, they don't need to make drastic improvements.
The Heat should stick with Chalmers. Nash is an unnecessary luxury. Three stars is enough.