The nice thing about iMessage, and reason I like to enable it, is ‘seamless’ integration of with all your devices, Mac PCs, iPads and iPhones. Your messages, be they SMS or iMessage, can be sent and received by whichever device happens to be most convenient.
I just think it needs to allow the user to more easily override Apple’s chosen message delivery vehicle, and explicitly choose between SMS and iMessage. Apple are not always good at understanding that, sometimes, the user knows best.