Just read an article by Felix Salmon in response to a decision by Gawker to stop pushing the full text in their RSS feeds.
The reason I prefer this is that I am probably one of the few people to use River of News approach to feed reading, which imho is the only rational way to read feeds.
My eyes are very good at scanning. I can quickly tell whether I need to read the full article. This allows me to consider orders of magnitude more stories than I would if I had to wade through feeds with full text.