I was just thinking the scenario and sharing thoughts. Once the UE gets handed over from source enodeb to target enodeb with MME relocation, target MME shall send paging message to UE with IMSI since S-TMSI was allocated by source MME and will be invalid at target MME. S-TMSI is a combination or MMEC+ M-TMSI, reason being, I am thinking this may the scenario when MME sends paging message to UE with IMSI.
It is just my thought, there may be other scenarios as well in which MME prefer IMSI in paging message. Looking for others to respond.