It's getting less common, but some people still pay exorbitant per-message rates for SMS. I no longer care, but when iMessage first came out I was really happy that I could finally text (some) people freely.
I'm floating in the same social circle right now. 99% of my friends and family have an apple device to communicate. I just kicked the text-messaging subscription bucket.
I only have unlimited texting because it ended up being cheaper for me to get a package that included it. It's nice to be able to sign up for SMS alerts and such without worrying about the cost, but if it hadn't been cheaper I'd still be relying on iMessage for everything.