Its not unhelpful its a fact everyone here will agree if you allow guests to post then most will be spam bots
I came here looking for solutions, not for opinions, but if you insist I must say I disagree with you, i allow guests to post in my forum and with good antispam measurements I have no problem with spam at all.
Sometimes people have something to say and they do not want to go thru the trouble of registering to a forum they might never come back, I know because I have been in that situation many times, i just pass on, haven't you?
I do not have problems with spam bots in my forum and guest are allowed to post. I also moderate their posts, but it is my choice and we are all very happy.