I think it's more "When you don't have a passing attack as good as the Patriots do...."
It's simply amazing what they've been able to do with the state of their running game (which is far from awful, but also not a big time one).
The people who say the Patriots have no running game or the "Where is Maroney? Is he hurt?" people don't know anything about football (not you btw, just saying). The guys on BCN were talking about this too. The Patriots rushing game is ranked 8th in the league out of 32 teams. It's better than 75% of the teams in the league. If they want to run the ball, then can. But with one of the best passing games in recent memory, they don't have to.
What they do depends on a ton of factors: who they're playing, gameplan, down, time in the game, position on the field, score of the game, etc. The Steelers have the #1 defense in the NFL, but they were missing a key part of the secondary because Polamalu is out. So you're going to try to take advantage of this weakness. You can't run on the Steelers because they're always keeping at least 7 guys in the box and are always blitzing. That's why their rush defense is ranked #1 in the league.
Brady & McDaniels drew up the game plan to exploit the Steelers weaknesses (Anthony Smith lolz, burned for 3 touchdowns) and it worked to perfection. Look at that perfectly executed play-action by Brady to get Smith to commit to the run. Moss ran right by him and got the 63 yard touchdown pass. Same thing on the flea-flicker.
Best part of this whole thing was Belichick after the game. He NEVER takes shots at the other team's players, but when asked about Smith he said "We've played against much better safeties than him."
Oh, and one other thing about Maroney: DID YOU SEE HIM MOVE THAT PILE 11 YARDS? OWNAGE. He's got the skills and if they want/need to, they can utilize them.