To play Devil's advocate, surely the reason people use MMS is that up until pretty recently it's been impossible to send emails with photos attached from a 'phone? Isn't it a bit of a historical thing that will disappear as more 'phones and networks become better at doing email and data in general? After all, why MMS or SMS when you can email or IM for free?
Yes, I think that's the natural end point for this and agree that email is progress over MMS. But at the moment, we're going to very much be in the minority amongst the people I know. A lot of the older generation use MMS because its simple to use and you don't need to faff about with anything. if you know someone's mobile number, you can send them a picture. Add in an email address though and its just going to complicate things for them.