Like Christopher suggests, the money's got to come from somewhere. Or, as the saying goes, there's no such thing as a free lunch. I don't know the home builder in question, but a buyer really has to ask whether it makes sense to get a reduced rate for a year or two, then pay more--once the rate readjusts--on a larger mortgage. In times of steady or slightly rising prices, it can make sense. In times of declining prices, you may just find yourself stuck with a home worth substantially less than you owe. And with the Fed likely to continue cutting rates, those "super low" rates today may not be so "super" tomorrow.
Just a thought.