It is often cost prohibitive to put the full text in the feed because of all the clients that neither handle the inbuilt TTL value, nor use http caching correctly. Large (in terms of bytes) RSS feeds are one of the key places where etags and conditional requests are useful but many clients just ignore all that and repeatedly request the whole feed.

This sounds like it couldn't possibly matter but it's actually quite easy for a full text RSS feed to be the vast majority of bandwidth for a site.