iMessage will only work if you are trying to text to another iOS device. What if the other device does not have iOS (most do not have iOS)? What if you don't know if the other side has iOS? Wireless Carriers offer a ubiquitous texting service...which is easy to use, always on, and works with any texting device.
Apple is trying to lock in people to their architecture.....just like Google is. This is starting to get dangerous.