That's one of the things I like about this complaint - it points out that they don't allow any other apps to support SMS, so only iMessage has the ability to message anyone with just a phone number, seamlessly upgrading if the other party has iMessage and using SMS otherwise.

It's not solely about iMessage not being open, it's about reserving key features for only iMessage to give it a significant advantage. (Also mentioned are other key bits like running in the background, etc)