Mostly it's a good thing to let bots spider the forum because then it will show up on more search engines and increase traffic and the member count. Spiders mindlessly follow links so it will appear that they're up to no good but usually it's nothing to worry about. Other bots are currently scouring the net for copies of copyright protected text or comparing student's coursework etc against online documents. Then there are bots which are unleashed onto the net for data gathering/research purposes. Those are just a waste of bandwidth and can be blocked.
My forum has been indexed at
www.boardreader.com for a while now and it's bringing in a lot of new members. This is the Google of the forum world. If the info you require is out there on an UBB or vB forum Board Reader will track it down.