I post when I feel I can contribute. But remember, a lot of the "guests" are folks finding the site from places like Google/Bing. They're far less likely to register and really "join", and more likely to simply find the answer to the search they made and move on.
Depending on the forum software, "guests" can also be shown for the various web crawlers out there that build indexes for sites like Google/Bing. Sometimes they're shown in a bot category, but sometimes they're lumped in with guests. And while it doesn't seem like there would be many, you'd be surprised how often they hit a continuously evolving, changing, and growing site like a community forum.