Historically speaking the 2-3-2 offers a great advantage to the team with homecourt advantage. In the playoffs, much different than the regular season, it is about adjustments from game to game. Protecting home court against a good team is tough as we have seen throughout these playoffs. Having to do it three games in a row is nearly impossible and negates some of the advantages of striking back with adjustments.
Dallas has to play all three at home in a row and throwing everything out the window (stats, matchups, etc) it is nearly impossible to win 3 in a row once you get to the Finals. You can pretty much guarantee one Heat win out of those three without any kind of analysis, the pressure to hold court three games in a row is intense. If the Heat come to play, they will get two games out of it.
I'm sure much of it is psychological. After a team loses in the playoffs on the road, they just say "we are just hoping for a split" or something like that. If they lose both they say, "we just have to get a split next time". The pressure always rests on the home team to protect the home court. The pressure of doing it three times in a row against a team good enough to make it to the Finals is huge, not to mention the difficulty of beating a good team three times in a row.
Since both teams are great road teams, the disadvantage isn't so much that the Heat can play 4 at home and the Mavs can only play three. It is that the 3 for the Mavs are in a row.