The killer feature is that you know when the message was opened. With SMS/text-messaging you have no idea if it was received by anyone - since it might have been sent to a landline #.
It's interesting that in North America it was an intentional decision to make land and mobile #'s indistinguishable, yet we are now supposed to keep track. I'm probably a bit sloppy with my contact list, but still people forward calls to their mobiles or transfer numbers. I've definitely sent SMS messages to the wrong place before - also because people change their mobile numbers frequently.
