Dear Dag.
I don't understand. Without the (upstream) RHEL RHN client in CentOS people are forced to use another repository, even when the RHN client would be sufficient.
One disadvantage of using RHN(S) to manage CentOS machines is that you cannot really manage EPEL with it as it contains some 'duplicate' packages which are already in the Red Hat Tools channel.
Sure there are benefits, if you'd be interested to use CentOS with the corporate RHN Satellite (to keep RHEL+CentOS infrastructure identical). Or simply to bootstrap from RHN/Spacewalk to download/install the updated spacewalk client ?
Why not ?
If the packages are going to be incompatible one time, you cannot even register your system to Spacewalk.
Best Regards Marcus