Update: WordPress staff have since added a word count at the end of the sample in the Reader. If you click on the word count at the end of the sample, it will take you directly to the blogger’s website, rather than opening the post in the Reader.
What’s the advantage of doing this? That way, you won’t have to click twice to read one post. Some bloggers allow you to read the full post in the Reader, others set it so that you can only read part of the post in the Reader. In the latter case, if you click once to open the post in the Reader, you’ll need to click a second time to visit the blog and read the rest of the post.
So just clicking the word count to visit the post on the blog, rather than in the Reader, saves a click. If you have hundreds of posts in your Reader, this can save a lot of extra clicking.
(This post originally had a suggestion for allowing readers to go straight to the post on your blog from the Reader, but now that WordPress has added the word count link at the end of the sample in the Reader, that work-around is no longer necessary.)