Really useful for blocking unwanted web fetches while I am inside any Internet Banking Secure sites Eg., block www.google-analytics.com and the same applies to the sharemarket logons, I dont NEED or WANT a facebook icon while I am doing business!!
Another use is browsing from behind the Great China Firewall. Many sites have facebook and google icon/links type "fetches", these cause pages never to complete, Eg., browsing Western news sites, or even StackExchange and GitHub.
As measured by WebMIn (visually) ...about 2.25Gig RAM Released by FireFox on an 8Gig Linux workstation.