One of the biggest reasons to do partial feeds is to avoid splogging. Another popular reason to use feeds only for a peep is to encourage site visits, which in turn might increase the advertising revenues. Scoble has led a debate on this earlier.
Me? I support full feeds. I think there are better solutions to the problems. Feed advertising can solve problems for some who think full feeds is anti-revenue. There are multiple solutions like Feedburner or Pheedo. Splogging can be dealt with approaches like AntiLeech or reporting them or this guide from Lorelle against content theft.
Some think that reading posts on the blog itself can enhance the overall experience. I think it is best left to the personal preference of your readers, rather than forcing them to visit the blog.
I have signed the petition, go ahead and support it if you think full feeds will help.