The "appeal" of iMessage is that it's basically just integrated with regular text messaging so the user doesn't have to even think about how the messaging is done and simply use one Messages app which will on its own determine when to use iMessage or regular text without the user giving it much thought. It also works in conjunction of being able to receive and send messages from and to same conversations on other iOS devices and Macs.
As far as replying to a message, you can still basically do what you described in iOS as well. When a message comes in and you see a notification you can pull it down to reply right then and there and stay where you were prior to the message or you can pull down notification center after the fact and swipe or 3D Touch the notification to also reply to it right there without leaving the app you were in. That goes for many other notifications for other apps as well (if the app developers updated them to support that).
