Most search spiders have been known to get a bit crawl happy from time to time. But the most complaints over time come from MSNBot which tends to often get out of hand and send their spiders on individuals sites are rampant rates. What is the issue if spiders are crawling your site? Well, no issue unless they crawl your site at a point where it hurts your server and resources. Spiders need to tame themselves and behave nicely to the web servers, which MSNBot has been known, on occasion, not to do.
I decided to check our forums here and noticed that of the 23 search bots active on my forum, 19 of them were from Bing, MSNBot and 2 were GoogleBot and the final 2 were Yahoo Slurp. So, of the spiders on my forum, about 82% of them were MSNBot. Others are reporting similar numbers or worse.
Forum discussion at WebmasterWorld.