I think this helps and hurts both teams.
First, the Suns. Yes, they've got the presence inside that they've sorely lacked for years, but what kind of presence will it be? There's no timetable for Shaq to return from his hip injury [honestly, how did he pass his physical?] and when he does, he'll have to get back into shape, learn the Suns' offense and possibly guard the likes of Tim Duncan, Yao Ming and Dirk Nowitzki, who are all younger, faster and more agile that Shaq is at this stage. Plus, the Suns are a run and gun team. Can Shaq maintain that pace for the rest of the year? I highly doubt it. And if he goes down right before the playoffs, what then? They don't have Shawn Marion anymore to pick up any scoring lapses and Amare Stoudamire certainly is not a defensive force. The Suns will be right back where they started.
Now, the Heat. Good job! You've got another NBA ego-driven, "I'm disrespected and unappreciated so just gimme the ball and my money" player to go on your roster. How is Shawn Marion going to deal with Dwayne Wade, who has had to become a score first, pass second kind of guard? Not to mention the fact that Marion has not been the first or second option for the Suns. He was often option 3 or would score off offensive rebounds or busted plays or fast breaks. Now, he's going to be asked to do that PLUS be an additional scoring threat PLUS play defense against the Paul Pierces of the East. Marion has made it ABUNDANTLY clear that he wants to get paid. Can he put those desires on the backburner long enough to help Miami get back in the East playoff hunt? And if they fail to be successful for whatever reason, is he just going to bail on them at the end of the season or will he give them a chance to re-sign him? If he's just planning to bail, which from what I read seems to be his mindset, then Miami has just wasted their time bringing him over.