Also, the search engines make cached copy of a page so for example there was a thread on these forums that got deleted, you might still be able to view some of it in the search engine's index
It's good to have search engines crawling because if someone needs help with messenger plus! and they haven't notice a link to the forums say in the "plus! about window", then they go to google and type "messenger plus help", boom, msghelp.net appears
I think that the ability to see when a bot is seaching something is good, so you can filter out the number of bots who are the guests viewing the forums.