in addition to everthing that's already been said here:

MMS, like SMS, will make my phone ring/beep/vibrate/... immediately, right after the sender hit the button. email would need to be push-email for this. now given the fact that many handsets don't even support email (or the owner never configured email properly), how many do push email??

it's a different kind of content that goes into MMS. things I want to share now and that have no relevance at all 20mins later.

example?

a friend of mine is single and searching - searching so hard we can't help but make fun of it. - the other week i spotted a group of men in a café that i thought would be interesting for her, so I sent her some pictures by MMS. because it was MMS, she received it instantly and asked for a close up of one of the guys. she got that plus a short video. then she noticed that there was another guy at a table behind them, and she got another picture. etc. etc.

we must have exchanged ~30-50 messages this afternoon, at least 10 of which were MMS with images/videos attached.

that wouldn't have been possible with mail - given that she wasn't home and doesn't have push-mail, she would have seen my first mail only hours later. no way to say "oh, please give me a close-up" then.

it's a different quality. it's not only that you get the data through, it's also about when and how it's presented at the other end.