This isn't a full answer, but I know of a few "smart" appliances out there that use these gateways to deliver messages. The washing machines in my college apartment (which were otherwise extremely dumb) did for some reason.

The amount of traffic that goes through these gateways probably isn't worth charging for, given the amount of headache it would cause for incidental uses like the above. But it would probably cut down on gateway spam, which seems to have increased lately (at least on AT&T).