I agree with czsz. Leave it as is, if people want to register, they can. If they just want to visit, or are a bot, then let them. Although, we've had people in the past claim to be intimidated, and waiting to post after they'd been a 'guest' a good, long time. Plenty of the "long-time readers, first-time posters." It has seemed like we've had an influx of newer members recently, or maybe just members who have started contributing regularly. Shepard, Sicilian, to name a couple off the top of my head.
One of the reasons I like this forum is that there really does seem to be some identity behind each avatar. Amongst those who do post regularly, personalities are easily perceived (whether or not they are honest) and banter between regular posters is fairly, unique? The best way to say it is that if the screen names were hidden, I bet that posters could identify at least a few of the members based on their words.