Keeping kernel 2.6.18 as a base (read the same API) does have its advantages in the some commercial places. One of the suckiest thing about Fedora for me personally is running binary nvidia drivers which break often when new kernels come out.
By contrast, we maintain a commercial telephony IVR based on RHEL3. Its still in production, still works, and the company that owns it has no interest to update, with the exception of security updates. Due to redhat maintaining the same kernel release I was able to install yum on this machine, "yum update" over 600 packages (including the kernel), reboot and have the machine come straight back up, and start taking phone calls even without rebuilding the proprietry drivers for the IVR hardware.
A lot of purists will hate that, but in an unbiased world where you just want it to work (enterprise), it can be a major plus.
Even so we are running RHEL 3, 4 and 5 systems now and Im looking forward to RHEL6 for new installs. The old ones wont be updated so long as they still serve their purpose.