The service node's OS itself is/can-be stateless, but you still need to either laboriously copy the /install data every boot or else mount it in the running image somehow. So it's easy to provision them but you still need some stable data storage for them. The "stateless" xcat stuff is mostly aimed at compute-side operations where everything has a shared filesystem anyway.
I'm familiar with rocks, but I've never seen a warewulf cluster in the wild -- are you still using it?
To be fair, Warewulf largely 'suffered' from the same issue -- you could pack a fat initrd to be delivered over the network, but doing that at every boot with the full stack needed for compute at the time was crazy slow, and in some hardware configurations just wouldn't work. In an HPC environment we had shared filesystems out the wazoo (and with some amazing performance), so it made sense to mount up the majority of the os filesystem over NFS (and I wanna say some creative use of overlays).
I was only with the group a couple years nearly a decade ago (my job is up in the clouds these days) but it looks like Warewulf is still actively used: http://metacluster.lbl.gov/warewulf
I'm familiar with rocks, but I've never seen a warewulf cluster in the wild -- are you still using it?