If you have closed-source applications installed like Opera (I do), Skype, or whatever than AppArmor should be engaged. Especially for Skype Linux it’s irresponsible without it, since Skype Linux reads
/etc/passwd, Firefox profile and other files. For Ubuntu Feisty it’s meant to be in Universe and in Gutsy it will be installed on default (without profiles, though). On the community help there is an instruction on how to install and use it for Feisty and Gutsy.