Well it's a little different because a lot of people, even if they have email on their phones, don't have push email either. SMS is nice in the sense that, because it's part of every phone, it's pretty much guaranteed to be an immediate notification unless said person has their phone off. That's something that email can't replace any time soon.
I agree that the prices for SMS and MMS messaging are outrageous for what they are though. I pay $30/month for 6GB of data, and you're trying to tell me that I should be paying 15c to send 140 bytes of data? It's crazy.