Kev Quirk wrote a great article about why having “full post RSS feeds” is a good idea. It’s mainly because then people can use a feed reader, customize it to their personal preferences (font, contrast and other accessibility features) and read the content in their preferred way.
You may have readers that have special needs, but want to read your content. Maybe they need a high contrast ratio, much larger font, or a screen reader plugin.
By using a feed reader, they can setup a more accessible environment once, then subscribe to feeds and have the same experience across all sites that they read.
As a blogger what motivates me to provide a full content feed is that it doesn’t really matter how people read my content, but that people read it (and ideally also find it useful or interesting). I don’t need or want to squeeze my readers for dollars (or euros), but I want to share my thoughts.
But what I already noticed is that you can usually tell from the feed of a blog if the blogger subscribes to feeds as well. If, for example, a blog has no feed at all or just a feed with excerpts, you can usually conclude that the blog owner doesn’t subscribe to any feeds him/herself and is therefore not aware of it.