In the news today: a new exploit to hijack your PC was found - it uses Flash banner ads distributed by Doubleclick to grab your PC. It utilizes Flash and doesn't even have to be "clicked"!
Article on Wired: http://www.wired.com/techbiz/media/news/2007/11/doubleclick goes into detail.
Apart from just plan not wanting to see ugly ads, this is another reason that I don't have flash installed.