http://www.blackviper.com/windows-services/dns-client/
"Only in extreme situations should you disable this service as caching DNS lookups reduces network traffic and makes internet surfing performance faster."
With a reasonably fast network you may well not notice the difference. But, as I said before, Vista was designed in the days of slower networks; and much of the world still has to put up with such a situation. Also, it is just good netiquette not to needlessly squander someone else's (the DNS server's host) resources (the DNS server). Anything that reduces network traffic, and the load on upline DNS servers, can only be a good thing.