Some are innocent misuses of guest book entries; some are intentional with goals to advertise some product or service.
One thing that bugs me is the persistent blog "comments" that appear as some unintelligible language, font and purpose except to post an obscure link for unwary readers to follow.
Get a life, you jerks. Stop trying to prove something that you can mess up another’s online experiences, push your crappy products, services or ideologies on people.
Go read a book.