Of course a lot of users will "abuse" their privilege of posting anonimously in order to be assholes. That's kind of to be expected. On the other hand, you can freely speak your mind and that freedom does not only mean to be an asshole.
Contrary to register-only boards, you don't have to regard other people so much when you speak your mind for fearing you will lose your reputation or friends or whatever. And even if you aren't afraid of that, it's probably still better to post anonimously when speaking your mind freely about some controversial issue or when you take a controversial stance because when other people take offense to that you won't have to deal with ad hominem drama in the following discourse.

Also, it's just easy to make a comment anonimously. No need to register, no need to have an e-mail adress for that, no need to remember passwords. It doesn't get much easier than this.