There are many advantages to upgrades: better performance, it can take a while before drivers are backported to old kernels so you can't use that shiny new server.
Of course, some people then add EPEL to their RHEL installation to get things like new kernel versions; to some extent this defeats the whole point of having an "appliance-like" OS install.
That's basically wrong. It's been perfectly possible to upgrade from fedora 18 to 19 to 20, and indeed I have, as have many colleagues. In contrast, upgrading from centos/rhel5.x to 6.x is far from trivial.Most other distros like Fedora, etc. only update for a year or so, then they expect you to throw it all away and install the next version. They don't even offer an upgrade capability, you have to format your drive and reinstall the new version from scratch.