Miami is not what we would consider a "deep" team, and is less so with the recent Chris Bosh injury. In Bosh's stead, Mario Chalmers and Udonis Haslem have had some crucial contributions. Haslem, most notably, drained late shots against the Pacers in Game 4 of that series. Chalmers had the game of his life during, well, the game of Rajon Rondo's life. While we'll always remember Rajon's 44 points in 52 minutes, we may forget about Mario's important 22 points on 16 shots.
The Heat would be in a tough spot were it not for both players, but Haslem is more crucial to what Miami needs, especially if Chris Bosh continues to sit nattily attired. The Heat are water-color thin in the front court, it's an obvious point of weakness. It's not just about lacking rebounds or inside presence, though Bosh's absence leads to a dearth of both.
What Haslem, if playing well, provides is the ability to draw a defense out. It's what helped Miami win a 2006 Finals when opposing coaches employed the "Hasleming" strategy of directing all the help towards Wade. Udonis has made a career of nailing the mid range jumper, and if defenses are respecting it, then Miami's perimeter superstars can run a decent pick and roll.
It's quite valuable for Haslem to draw a defender far from the hoop upon setting a pick for the ball handler. If Chalmers is playing well, he's just playing well. If Udonis is playing well, he's opening up space for Dwayne Wade and LeBron James. Space is needed by both with so much defensive attention on the Big Two. This makes Haslem the most important third player currently dressed.