I personally don't find wiki guests a good idea; may leave the door wide open for vandals.
I wouldn't worry about that. Sure, you might
get vandalism, but the chances of that are pretty slim. This is neither a particularly large nor controversial wiki; no offense, but do you really think anyone would care enough to vandalize it, let alone have any reason to?
Besides, I think you're underestimating how lazy the typical internet user is. Making an account really doesn't take much effort, but it's a psychological barrier that would stop most people from making a minor contribution. "Oh, I can add this thing — I have to sign in? Ugh, fuck it, someone else will do it...". The same principle that keeps vandals away ends up having the same effect on potential contributors, too.
Of course, the pool of potential anonymous contributors probably isn't any bigger than the pool of potential vandals, but either way, what's the worst that can happen if the wiki is opened to guests? A vandal shows up, a few pages have to be rolled back to previous versions, it gets made users-only again, end of experiment. Why not give it a try?
captchas for guest posting? And name, and email? Damn, son, maybe it actually would be less work to sign up.