Why do so many people find it so hard to understand that MMS and email are NOT the same damn thing and has a completely different usage. It's like saying "why send a text message when you can send an email?".
In the UK at least, MMS (known as 'picture messaging' to a lot of people here) is a huge thing in the same way that standard SMS text messaging is. So, I have to agree that omitting such a basic feature would be a mistake.
As I said above, the systems are somewhat redundant.
Imagine a hypothetical scenario where everybody has an iPhone. At that point both SMS and MMS are somewhat useless. If sending and receiving an email is just as easy, why have some separate system just for your phone to accomplish the same thing? (Assuming there is no significant lag in sending email compared to SMS/MMS).
The problem isn't the concept, it's that Apple is up against tradition and compatibility. It's a fight they sometimes win and sometimes lose, so we'll see. Perhaps they are betting the cell phone industry is moving more towards email based messaging.
Still, in the short term, they will get a lot of flak for no MMS. It can always be added if the backlash is too great. I'm also hoping they add instant messaging in the not-too-distant future.
The iChat-like SMS system loooks awesome, however. At least when they DO a feature, they do it right.