Sites devoted about sharing contests, like My Blog Contest and The Prize Blog, features more and more blog contests.
The usual blog contests features these few requirements:
1. Blog about the contest with specific anchor text
2. Blog about the blog holding the contest with specific anchor text
3. Comment or use some other form of communication to let the contest creator know

Usually, a blog contest means:
1. More visitors
2. Better ranking in search engines for some specific keywords
3. Some visitors might subscribe to the blog

A huge amount of benefit can be brought of the contest starter.
How about the contestants?
How about the Internet?

The contestants spend one post just to describe a contest to gain a small chance in winning some small price. If the post is unrelated to their blog, some readers of the contestant's blog will not be happy.

The Internet is created for information1. The blog contests is a way to play with the search engine. Contests are bribing bloggers to review their blog, like PayPerPost. Even a small blog with no much potential in some keyword but with high incentive prize, it can be increased in search engine rank. Thus the readers will read some poor quality blog because the blog paid some money for it.
PayPerPost users got punished by Google. Google will figure out a way of attack blog contests.

1. 1. I know the saying "Internet is for porn" but porn is information.