Why PMIP is being used in S5/S8 interface

I know this is not the right time to write about the LTE stuff here, because we are still in the UMTS phase, but I found it really interesting!

Proxy Mobile IP is being used for the CDMA and Wimax stuff for the communication to UE, where a Mobility Access Gateway (MAG) communicates with a Home Agent termed as Local Mobility Anchor (LMA) for the UE database update and Routing area update and Location area update!

So why we are giving this space in LTE as well?

That's the question that was in my mind!
Now suppose a CDMA or Wimax device is entering into a LTE space, or you can say a handover is taking place.

How the handover will take place if no entity in the LTE even understand the PMIP for transfering the traffic.

Now this is what I got from the 3GPP TS 23.401
section: 4.3.8.2 Serving GW selection function

"If a subscriber of a GTP only network roams into a PMIP network, the PDN GWs selected for local breakout support
the PMIP protocol, while PDN GWs for home routed traffic use GTP. This means the Serving GW selected for such
subscribers may need to support both GTP and PMIP, so that it is possible to set up both local breakout and home
routed sessions for these subscribers. For a Serving GW supporting both GTP and PMIP, the MME/SGSN should
indicate the Serving GW which protocol should be used over S5/S8 interface."

This clearly means that the PMIP is providing a clear support for CDMA and Wimax UE during handovers. Later point of time I will try to uncover the complete high level message exchange during this handover.

Hope to get some comments!
in reference to: 3GPP n LTE (view on Google Sidewiki)