I do..they are under nofollow tags so won't count in seo backlink on google Your readers can know who references your post.That's good aproving it..
Most, though I dofollow them, it'd be rude not to. Some I don't if they're quite obviously scrapers though.
@diego018: mostly it's just that - write a post with a link to a post on another blog. This process is automatic if both blogs are running on Wordpress. With other blog systems, there may be an intermediate step involved.
This is good for one's blog cause when you are linked from a blog that already has good rankings then your blog will benefit from that.