I look at the recent history of the anti-abortion movement, and it seems to me that having a good-zealot/bad-zealot strategy has been highly effective. Oh, sure, abortion is still legal and the White House and Congress are run by the (nominally) pro-choice party. But abortion rights are in a slow but inexorable retreat. What's more, the actions of of the "bad zealots" -- shooting abortion providers and whatnot -- are now treated by conventional-wisdom purveyors as stuff done by the marginal bad people, who are not to be confused with the fine, virtuous, respectable anti-choice folks in the C.W. purveyors' Rolodexes. How does that work out for the movement? Well, the shooting of George Tiller didn't exactly slow down Bart Stupak, did it?
Fox and the GOP are hard at work nurturing what they hope we'll come to see as the mainstream of the tea party movement -- just a bunch of nice patriots who simply want Republicans to act more like true conservatives. To someone who takes a close look, it might not be easy to tell where the "good zealots" and and the gun-toting, conspiracy-mongering "bad zealots" begin -- but the hope is that casual observers (and the mainstream media) will accept the argument that any ugliness arising from the movement is from the discredited, unrepresentative fringe-dwellers.
It's always useful for a movement to have more extreme elements to whom the moderates can point and say, "Deal with us or you'll have to deal with them."
But it says something about the state of the "center" in American politics today that it takes someone who actually shoots people in order to make an anti-choice figure like Stupak look moderate.