It did see a lot of use. I forgot to say, but the majority of 3G UMTS and 4G LTE radios (operator level and even down to tiny things like femtocells) required it to connect back to the $(core network) and encapsulated a mix of control and user data through it (though often also encapsulating SCTP in a VPN). That is to say, there were a lot of devices out there that used it in anger. If you made a call or used a cellular data session for over a decade, your traffic was likely getting encapsulated through it.