Both S5 and S8 can be either PMIP or GTP. Courtesy this link (http://3gpp-n-lte.blogspot.in/2010/02/why-pmip-is-being-used-in-s5s8.html), this is what I understood.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(my guess). 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