Michael Eisenberg, a VC and nice guy, thinks that it doesn't make sense to share your content in full, in your RSS feeds.
Michael, understandably, comes at the question from the business side and discusses his feelings on monetization, and even how RSS itself is a fad.
I have a very different set of beliefs. There are a couple of simple reasons why I have always uses full entry content in my feeds:
Give your users what they want
If a user wants to read my content in full, in a feed reader. More power to them. I always want to extend my reach, and offering content in different form factors makes sense to me. Some people like to read everything on my website directly, others aggregate on My Yahoo!, others still read away on their feed readers. I am sitting on a plane right now, and due to my feed reader, I can spend that time flirting with the mass of information that lies deep in my reader. Instead of the usual quick dash to find the best content, I have time to delve into that second or third tier of feeds that I often overlook. It beats watching the kid in front of me screaming!
Compete against yourself
If a lot of your users prefer reading your content via your feed it means that your site isn't giving them what they want. Having your site compete with a feed reader view of your content is good in that you have a baseline to go against. It will make you add more value to the site itself... and hopefully most people will want to hit the site itself.
You will be careful not to flood your web viewer with ads quite so much too ;)
Tweak your RSS view
If there is a feature that people are loving on your site, consider adding it to the feed itself. There is no rule that says your feed has to be boring and totally stripped down. You can allow users to see comments, or add comments, directly from the feed view. This can keep the balance of the competition interesting.
You can also put ads in your feed just as easily as you can on the site, or in emails, so you can monetize it too. Why alienate the feed reading posse? Don't you want them to read your ads too?
Searching within your content
My feed reader is more than just content. It is a database of my trustworthy (or at least interesting) view of the web. I often run searches in my feed reader to slim down my results to this data set. In fact, I also have a process that created a Google Custom Search from my OPML file. As Google gets more and more personalized on search results maybe by giving it my OPML it could be a hint to setBoost(1.2) on content from these friends (which you could rank too).
New search engines
There are new search engines such as Allth.at aiming search not just at indexes, but searching via API (if they can find one), then RSS feed, then index. If you do not have a full RSS feed, the user isn't going to find your content.
Future of RSS
We could talk a lot about whether RSS will be mainstream, and whether your mum will be talking to you about Atom vs. RSS. I think that the core value of subscribing to content will be here for a long time. It may not (or will not) by called "RSS", but we will be consumers.
This is all why I will keep "giving away" my content. It is an honour that anyone even cares enough to reach out and subscribe. I respect them enough to give them what they want.