As someone who doesn't currently use an iPhone... isn't Messages the only approved SMS app for iOS? What happens if you attempt to message a non-iMessage user with that setting disabled?
When initiating a new conversation, the number you add to it appears blue if they can receive iMessages, green if not.
If for some reason iMessages can't be delivered to the receiving party at the moment. Text messages are used as a fallback for them. If you disable the fallback mode. It won't be.
The whole thing is kinda intuitive, and works really smooth IMHO.
Once someone knows about it or find out about charges, sure. But see how many people get hit with accidental roaming charges even though there's a simple and obvious setting to disable it.