In short PMIP is used when UE handedover from 3GPP network i.e. LTE to Non-3GPP network.
Some operators prefer using PMIP throughout their n/w as it facilitates connecting to non-3gpp accesses, whereas some operators want only GTP as their n/w is evolved from UMTS and they dont want too much hassles in managing different transport protocols. As a result of which, a user roaming between these two will not be supported home-routed traffic, he will only be local-breakout'ed. Therefore to support both home-routed and local breakout, S/PGWs have dual stacks.
Credit: http://tech.queryhome.com/7643/pmip-or-gtp-protocol