1) Improve user experience by allotting them the control to view the information which you share, in the way that works best for them.
2) Improve your relationship with your users. They will like you. Some will even love you.
3) Avoid losing your feed traffic as your users will not make a flight to substitute sites who are offering full text in their feeds.
4) Facilitate the development of a more mature RSS feed advertising model.
5) Save the world from inefficiency and wasted time by using RSS that is not crippled. Keep in mind it takes longer per a post for someone to have to click your excerpt, analyze it, determine whether it’s worth clicking through, wait for the page to load, then read the full post, then return to thir News Aggregator. It may be a 20 second expenditure, but that micro cost adds up when a person reads dozens and even hundreds of feeds.
Is the world better served by having every subscriber to your non-full text RSS feed spending 20 more seconds on every one of your posts or by spend 20 more seconds reading and analyzing more content? Think of all the positive externalities you could generate!
6) Get your content out to the users who have the least time and correspondingly, are the biggest informational relays. Per capita, these people consume more websites. These are the people least likely to keep a crippled feed on their blogroll, or at least, are less likely to pay attention to one. These are the people most likely to share interesting links/information/sites with people they know and get second and third order exposure for your site.
7) And because you will not make our list of shame.