Julian Sanchez, over at Andrew Sullivan's blog, has a post up that I didn't fully grasp until I read it a second time.
The shorter version (I think) is this: People who produce political opinion for a living (i.e. "pundits") may start out with the noblest of intentions. But informal social pressures make it increasingly difficult to deviate from the ideology they started out with. Thus over time they turn into partisan/ideological hacks -- defending "their side" not in earnestness but to keep from upsetting sympathizers while protecting the "opinion franchise" they've built up over time.
I think he hits the nail on the head. It has been incredibly painful for me to watch all those professional Bush-defenders rake their heads for creative defences in recent years. This is why I still read Andrew Sullivan religiously. He lost the majority of his readership when he turned against Bush in 2004. Sure, I didn't agree with most of the pro-Bush (and pro-war) rant he produced before that, and I still don't agree with him on many issues. But I know of no other writer with a comparably sized audience who has kept his integrity by "switching sides" like that.
Kudos also to Julian for recognizing it this early in his career. Let's hope he keeps it up.