I already do that, I whitelist all access to my server based on user agents and MSIE, FireFox and Opera get thru as well as 5 major search engines and EVERYTHING else is bounced.
No, I don't site around blocking things, they simply never get to my content in the first place unless I let them. I get a report of anything new trying to access my site and review who/what they are to see if I should let them in next time.
Only several major search engines and well behaved "browsers" are allowed.
Which is also why my web site checks all inbound IP's for an open proxy as well as blocking the known list, and I block accesses that come from many server hosting farms as well for the very fact people use cheap hosting to put various automated tasks and proxies online.
Basically, if it's coming from an IP address that humans don't use, like hosting farms, or where it's vulnerable like proxies, I block it.