> There's a difference between updating existing servers and installing new
> ones. That's why RHEL supports--and need to support--multiple major
Indeed, but if you decide you're going to support X versions of a distribution, your support window for any one of them necessarily decreases with the period of its release cycle. Choosing X depends on how much manpower you're willing to spend on the problem.
Each major release in a stable distribution is effectively a huge set of forks. The older the software gets, the harder it can become to maintain, which is multiplied by how many major releases you're maintaining. Inevitably this has an effect on which packages, and how many, Red Hat is willing to support. (I really disliked RHEL until I found out about EPEL.)
Don't get me wrong... I wish RHEL 6 would have come around sooner. Just so long as that doesn't force users off "old" software too quickly.