Well, I originally thought the spamming resulting from our activated guest posting was our problem (moderators AdamP and myself) and no one else's.
But I realize it's also a nuisance for users browsing the main forums portal, who see the 'there are new posts' sign next to the Teudogar forum, come here to check, and usually find the 'new post' they came to see is nothing but spam.
Sure, this spam will always be deleted after no more than a day or so (thank you AdamP! for your astounding reliability & persistance), but until then it may already have inconvenienced several dozen RPGCodex forum users - and this every day.
On the other hand, the potential inconvenience of having to register before being able to post would concern only one or two people per month (not-yet-registered guests), so from a utilitarian perspective I agree it makes sense to disable guest posting.
If, on top of this, we consider AdamP's probably enormous workload in deleting all that spam, I'd say, well, it's a pity but let's get rid of guest posting; it isn't worth all that trouble.