well, it costs little to use it (a process in its own security context has no more overhead than a process running normally; though chroot jails, as usual, require their own copies of filesystems). And if you need to run something as potentially buggy as sendmail or BIND (huge sources of security holes), you can run these in their own context, so if a cracker breaks in through them, they can't get at your machine proper (if it works as planned).
You could also run a firewall/gateway in a separate context, which would (in theory) be virtually as secure as a dedicated firewall machine, but a lot cheaper.
This does sound sweet...kind of turned Linux into a mainframe style operating system. I can't think of any way I'd want to use it, but it's certainly pretty smart.