The guy has the most unblemished conservative credentials. He's also quietly (or not so, depending on your perspective) built up a nice conservative PAC that has worked a large number of conservative politicians - many of whom owe some loyalty and are currently in Congress. I know it's too late now, but I sure do wish he'd consider hopping in, should Romney not win by default.
We basically need someone at the Convention to win the nomination, since Romney and his team are going to lay waste to anyone who stands between him and the nomination. They've already rolled out a lot of attacks on Santorum, and expect some more. "INCOMING!!!!!"
Santorum, I'd watch your back. And head. And wear some Kevlar. Romney ain't the saint he wants you to think he is.