A couple of my sites used to get a lot of spambot traffic, mostly using Xrumer and a bunch of well-known exploit attempts, including some dictionary attacks. Almost all of these attacks are against well-known PHP frameworks (Drupal, WordPress, PHPMyAdmin, etc). None of the sites use PHP, but the bots don't care.
Almost all of this traffic was from servers in Fujian Province in China or from servers on OVH Systems, so I blocked those IP ranges.
I'm not worried about any negative effects from blocking the Chinese servers, but is it likely that being a black hole to OVH will ever matter? The concern I have is that other legitimate sites might have crawlers/indexers/aggregators that could fail to find and include me and result in less traffic over the long term. It seems like a small possibility, but I have to wonder -- has anyone else seen significant negative impact to useful traffic after blocking swaths of useless traffic?