Why Fedora?

Exactly. For running things which don't need a boot environment, and don't need system daemons like hald and D-Bus running, VServer is pretty good. Running old GCCs falls into that category. (I hope it stays that way).

It's also fine if you already have your apps set up to use it, naturally.

When it's less inviting is when the problem is to keep some old app working that's currently on a real machine running some old distro. KVM is quite good at making that work with minimal effort.

I put mail serving (anything other than a simple configuration) in the latter category, because these days a mail server is a fairly complex affair and will run alongside a couple of spam filtering daemons and virus filtering daemon and some delivery program or other, maybe also user's procmails and perhaps involving the IMAP service too, and undoubtedly depends on a few non-obvious files in /etc as well as the obvious ones. That's the sort of thing I mean by "if I don't have time to touch this now, I can't afford to be updating the OS on this server to a newer version". Repeated prior experience tells me such things break on OS upgrades and can take a long time to get working again with the same behaviour as before.