Funny as it seems, this is what I think. They had that play on tap. One of the guys after the game (maybe McCourty? It was so late and I was trying to stay awake) said the Colts like to go low to block on extra points and FGs, so that was one of the reasons they used that play in this game. He said (almost as an aside), "Yeah it's too bad other teams will be looking for that play now." I think they figured if the Colts scored again they would try the extra point block to make it a 7 pt. game, rather than go for 2, which if they missed it, would make a later extra point block irrelevant. Since the Colts weren't playing great by then, there was also a good chance they wouldn't score again, so you could hold that play in your back pocket for another team that blocks low on XPs and FGs.