As per my knowledge, only HSS is used to keep subscriber data and UE location is updated in the HSS itself using the Update Location procedure from MME to HSS for an UE. So both the objectives (maintaining subscriber database and current location or serving network of UE) met just using single node. However another node known as SPR (Subscriber Profile Repository) has been defined by 3GPP, used by PCRF to retrieve the subscribed policies and charging rules.
Few techies believe SPR + HSS both will be combined as a single node for the deployment.
