Chris Pirillo thinks that partial feeds protect against spam usage of your content:

Heh. Yeah. That's what happens when you give 'em full RSS feeds, dude. Can't have it both ways, and I think you're finally seeing the frustration that many of us realized well over a year ago. I told you that full RSS feeds are idealistic, and that partial feeds are the easiest way to protect yourself from scraping and whatnot.

Umm, not so much. Scraping HTML is trivial - For sites I really want to read, but can't be bothered to visit in a browser, I create a local (file) feed. Then I add that to BottomFeeder. If you think the spammers aren't already doing that, you aren't thinking hard enough :)