Hi Geert,
I think there might be some reason, I have not tested and found any document; however here is my observation:
If endpoint is provisioned with CUCM and registered to it, then TMS will be getting endpoint details from CUCM
If CUCM-publisher fails and endpoint get provisioned and register to CUCM-sub, then in that case TMS can pull information from CUCM-sub if it is been added otherwise TMS won't be able to pull endpoint credential and hence won't be able to provide services including schedule conference and phonebook.
Thanks,
Vivek