Holy moly, Apple is very aware of the competing chat apps. iMessage looks amazing and they packed a lot of features in there, but I wonder how they're going to make it all easily discoverable.
I already find the SMS app on iOS confusing. Am I sending a paid SMS or a free message?
I dare say one of the major advantages of messaging apps in general is the certainty that the receiver will receive the message in the same shape as the sender send it.
With MMS you never know if the picture can be opened, and people just gave up.