Is it always desirable that you have to be a registered member of a forum to post a message? In most circumstances we want visitors to join as members (see converting the visitor into a member); we want them to engage with the site. BUT, what if your forum is on the sort of topic that a question gets asked, then answered and that is it. Maybe getting visitors engaged for this type of forum is not necessarily a goal.
For example, what about the Foot Health Forum, where people come and ask questions about their problem without registering as a member. Some registered members do answer the questions. The original asker of the question really has no need to register; they have no need to further engage with the site; they are unlikely to ever return to the site once they have the answer to their question. Any strategies to get these people to return are not going to work. The forum allows visitors to post without registering for that reason.
However, the forum can then become spam central and in the example above, it certainly is the case. Fortunately, the anti-spam measures, especially sending for moderation any post with a link in to a moderation queue means that almost none of it gets through (except for the usual non-bot human profile spammers).
While it is desirable to gain registered users on a forum, there may be some circumstances when making it easier to post a message by not going through the registration process may be advantageous. But be aware of the spam problem.