The bulk of mobile data traffic comes from services provided by parties like Facebook and Netflix, and these parties are Internet based. So what is the point of carrying data roaming traffic over a highly secure private IPX or GRX network, if most of it breaks out from the home network to the Public Internet anyway?
This is a very valid point. The standards certainly allow for local break-out of traffic by the visited network. But the implementation of local break-out has some challenges though, so for the time being mobile operators prefer the standard model. With growing data roaming traffic volumes the picture is change and local break-out is becoming more relevant and mobile operators will need to rethink their strategy. Local break-out will require decisions like: for which roaming partners and for which and roamer applications can a data session be allowed to break out locally?
Digital Interconnect is certainly working in that direction, aiming to reshape roaming such that the GRX costs do not grow faster than the roaming revenues.