I just added a page that list over 400 user agents that many web masters block for one reason or another. Every user agent listed is not necessarily a bad user agent. Some are from from small legitimate SEs. I think the bulk are from scrappers, and email harvesters.
I think I have explained on the page why I have included the different user agents. As time allows I will do research on each of the user agents and update the page.
You are the only one who can determine if you should block any user agent.
If you have comments on the list you can make them here.
You can view the list of user agents here.