|: Authentication might help prevent those posts, like the one the other day, that are randomly generated by a program. The program would also have to be able to generate a user name and password to get in, further complicating its' ability to post.
Absolutely correct. There are several benefits in implementing authentication and the one single disadvantage would be a slightly higher entry barrier (one could still allow unauthenticated post and just switch them off when there is a spam wave). I just won't enumerate all advantages that come to my mind or else the members of the forum will nag me to implement it. You can call me lazy for not having done it already, but it is a bit of work.