The "I'm a conservative , but..." preamble is worse than just Leftists who fear revealing who they are. They're liars, attempting to portray themselves as ideological allies with whomever conservative they happen to be attacking, in order to give the false impression that the conservative whom they are attacking is opposed from within his or her own ranks.
It's the lamest, most transparent tactic imaginable. It's like something a child would dream up. Or like the used car salesman who says, "Can I be honest with ya?" right before he tells you a boldfaced lie.