When you install FC6 test2 and select the Xen install option it does not install a native kernel. If Xen/XenLinux do not boot/work on your system you can not edit grub.conf to point to the native kernel and get a working system. This is a pain. A native kernel should always be installed.
This is intentional. If the kernel doesn't work, it needs to be fixed. Not papered over by having a different kernel installed.
Agreed, however when people with hardware that's not well tested on Xen come along and have an issue this will make their life more dificult. I also thought there were some cases, like requiring PAE, in which Xen/XenLinux were intentially not as functional as the native kernel.