I've seen quite a lot of tutorials that recommend you to ban user agents containing the strings libwww-perl and msnbot. I understand why one would ban libwww-perl, it's mainly if not only used for hacking and spamming.

But why are there so many sites recommending to ban msnbot/bingbot?
Since it's a search engine, even if only with a marginal market share, I would except one would want this bot to crawl one's sites.

What is it that msnbot does that makes people ban it?