Fullfeeds.com is “a petition against intentionally disabled feeds”:

Isn’t RSS about convenience? Wouldn’t you prefer to see entire texts in your feeds, rather than just summaries? Support the cause, sign the petition below.

While I’ve signed the petition, I’m not sure to what extent partial feeds are really deliberately used to drive subscribers to view the full post in its original context (and hence see the advertising), which would imply similar reasoning to splitting up articles to increase page views and forcing users to click through multiple ad pages to reach the file they want to download.
Certainly some bloggers will be using partial feeds for this reason, but equally, a lot of people who offer their feeds in a truncated format are perhaps doing so because their posts are longer/more involved and may seem ‘intimidating’ if displayed in full in a feed reader, especially if seen in a river of much shorter news items from other blogs – just as newspapers and magazines tend to have longer feature articles towards the middle and the second half, and shorter stories near the start.
There may also be plenty of bloggers who have simply not thought about the effect offering only partial feeds has. I know that I’m much less likely to read a post which is truncated when I come across it in Bloglines, simply because I can’t immediately see how long the post is, and hence how many minutes I’ll need to allocate in order to read and understand it fully (that makes it sound like I otherwise plan my time well, which is not true!).
So, although partial feeds can be an ‘architecture of control’ if used deliberately for forcing full views, I can’t believe that too many bloggers who actually use feed readers themselves would do it for that reason, because they must realise how annoying it can be.
Kevin Gamble and Stuart Brown have some interesting thoughts on this.