I run http://whirlpool.net.au and I religiously check the Amazon EC2 forum announcements[1] for new IP ranges to ban.
An excellent and very useful forum! It seems like whatever topic I'm searching for, google(.com.au) returns a useful result on your site.
Name me one good reason. Name me one.
Shitloads of rogue bots stealing content for black-hat SEO.
Shitloads of rogue bots harvesting email addresses.
Shitloads of rogue bots submitting spammy replies.
Those are bad reasons to close your site to all of AWS.
As nupark2 mentioned, there are legitimate users routing traffic through EC2, even some bots that you'd want to visit your site. Archive.org comes to mind (many of there scrapers are or were behind AWS). Closing your site or app to a large swath of the web is the wrong solution. It's like killing a spider with a bazooka.
(I also blacklist GWS, rackspace, linode, softlayer, reliablehosting, ovh.net, node4, netdirect, layer42, all TOR exits... it's actually a pretty huge list.)