Having the option of not registering is excellent. It allows for a chance to get a quick answer or post a quick message and be gone. Also, it makes one more anonymous.
Preventing anonymous users from posting prevents heavy spam and some specific individuals from coming back and flaming.. so having anonymous posting (where one does not have to register) is a very bad idea.
It does have problems like that, but there are a lot of benefits. Being able to be a guest allows you to be anonymous and does not give you a sense of obligation.
I think we have to bring in the risks/benefits analysis. I really think that blocking the IP is no good. I have some guy who continuously spams my blog and he has many different IP addresses. The risks clearly outweigh the gains.
Maybe people should be able to send a reply but it has to be accepted as fine. That's how they do it on Slashdot except for this would only apply to guests...
lolz yea it would be obvious if it was like "SPAM SPAM SPAM SPAM SPAM SPAM SPAM FREE IPODS FOR FREE JUST SIGN UP ON THE FORUMS SPAM SPAM SPAM SPAM" but most posts go to the hardware and software section