If you allow guests to view forums, you may have fewer inactive accounts(user names). I'm sure there are a bunch of user accounts that have been opened after the initial day because they wanted to look at something. . .
I say yes. View but not post. Before people were required to sign up so they could even view the forums, we didn't have nearly as much stupid shit posted. I think once they've signed up and become a "member" they feeled compelled to post something, anything... for some idiotic reason.
I don't think you should, if there's a spammer or something then you can't do much about it but if he/she is a member you can kick them off the board. Cuz im telling you IP bans do not work to get rid of pieces of shit spammers, or anything else, but do what you must either way its still a great site .