Forgot I have Ask whitelisted too but not Looksmart.
Before I started building the whitelist I looked to see who had been sending me traffic over the last 12 months and any SE with no meaningful ROI for the crawl got dropped off the list.
It's an easy judgement call if you give up 40,000 pages a month to a SE and only get 3 visitors a month in return. Blocking that crawler is a real no-brainer in my mind compared to allowing other crawlers that may send you hundreds or thousands of visitors a day in return for the crawl.
Yup, it's the only way to lock your content down as the SEs can't even stop bots hitting them appearing to be random from a series of anonymous proxy servers. That's why I'm also on a vendetta to stop all access by anonymous proxy servers I can detect as well as I've noticed crawling ping-pong'ing between several IPs that aren't even closely related and it turned out to be anonymous proxies.
Besides, leaving old cache content on SEs exposes your site in other ways that I'd prefer not getting into on this thread, that's a whole different debate.