If google were to index javascript, they would probably create their own interpreter which only interpreted content that was meaningful to a search engine.
If googlebot had to interpret every fade-in menu and every roll-over effect it would take substantially more resources for google to crawl the web. Googlebot would also be vulnerable to malicious scripts - or scripts built to waste its time.
twofish writes: "Google's YouTube video site will soon be showing content from the BBC in a deal announced today. Content will include adverts, clips from popular programs and news items. The deal is likely to be controversial, particularly since the BBC is paid for by a compulsory tax system (the license fee) rather than through advertising or subscription"