Well, please educate me. All I hear from advocates is "more security" without a concrete example. You mentioned the danger of emails get stolen without SELinux. Please give me the scenario. So we can gauge the risk.
Simple example. Daemons running under selinux can only access the things they are expected to be accessing. So if I was to crack your httpd and try and execute a shell SELinux would block it. Standard file permissions have no notion of who is doing the action and in what context so would not save you.
The biggest win for a lot of folks is web stuff. If I find a bug in a PHP script (which for most PHP is pretty much a given) that allows me to access arbitary files it will be a lot less useful under SELinux because only files labelled as http content can be accessed this way.
If I manage to use a bug to add a new script to your machine via the web server I'll not be able to get it to run as a cgi because the web server isn't allowed to create cgi binaries. Again won't happen with just file permissions.
Alan