Whether you have just opened your forum or whether you have an established community, the thought has probably crossed your mind at one point or another of opening the board to guest posters.
Obviously there can be no definitive right or wrong answer for everyone, but for most people the simple answer is “don’t do it”.
Guest posting is “very 2002″… I pick that year because that was the last time I saw a large, successful community allow guests to post. Since then, most have come to the realisation that guest posters can be a nuisance. The biggest problem since 2002, as many administrators will attest to, has been the spambots. Bots exploited phpBB2’s guest posting permissions in particular because of the lack of visual confirmation (captcha’s), and I’m sure that when the bots began to attack forums en masse was when many administrators decided to disable the guest posting permissions. phpBB3 thankfully has not had a problem with spambots yet, but after several years of making do without guest posters, not many forums would have been quick to re-enable the function. This has led to a chain reaction, as new administrators would have been visiting these existing “guest-less” forums for several years and in a subconcious attempt to mimic other boards success have refrained from allowing guest posters themselves.
The other problem with guest posters, not quite as large scale as the spambots but just as annoying, are the tendency they have to “troll” forums. In forum terms, a troll is someone who posts material to intentionally insult others, stir people up, and overall make the forum experience for others less enjoyable. Because guest posters don’t need to supply an email address, location, interests and other fields that would normally be filled upon registration, they can troll very easily and quickly and then disappear with the same speed.
Generally, a forum that requires registration will weed out troublesome users more than a board that is open to everyone. The registration process can be a psychological barrier, many trolls will decide it simply isn’t worth the time it takes to register and move along while those that are genuinely interested in the subject matter will be happy to spend a few minutes signing up.
Having said all that, guest posting can in some cases be a useful addition to a board. Not all guests will be looking to plug their latest pharmaceuticals or start a flame war with the forum regs. Perhaps your website is seasonal (only open for an annual event for instance) or will be open for only a limited time (for a one-off gig, event, etc)… in cases like these, where there is little incentive to register for an account that you may never use again, guest posting may be pivotal to the short term success of your community.