If you use a web browser to view more than a short list of trusted sites, you need selinux.
If you run network services accessible from outside the machine then you need selinux.
If you run binaries from semitrusted groups (this includes most commercial software) then you need selinux.
You don't _need_ SELinux in any such cases.
I wouldn't dare run some of the web plugins without them being very very constrained by a security tool. I'm not sure I trust some of the image libraries either although the google audit work seems to be slowly improving it.
Unfortunately application library security has taken a nasty turn for the worse because any library exploit in a library also used on the iphone is now being sat on by jailbreakers rather than reported.
Alan