Being able to send pictures and so on via messaging is standard on almost every other phone. There is a clear need for it. I do miss it on my phone. SMS and MMS are widely and currently used by a lot of people.
To me it's Apple hubris, which as a company they are guilty of doing many times. It almost killed them in the mid to late 90s. Can you imagine if Ford started selling a car without AM radio. Yeah it's not as popular as FM, XM/Sirius or recorded music, but there's no logical reason for no AM radio in a car. It's like apple not having FM at least on the ipod. Why? Apple hubris. I mean the walkman had am/fm radio and that was part of the joy of it.
Apple doesn't want to follow the crowd, it wants to lead it. That is why we don't have floppy drives on our computers today, and I daresay that is why USB flash drives are popular. At the time of the walkman, you could only play like 60 minutes(?) of music and to get more you'd have to carry around multiple cassette tapes so yeah radio back then was a relief when you didn't want to hear what you brought along with you. The iPod however now has massive storage so you are in fact your own radio station.
Anyway, back to MMS vs email.
Technologicaly, MMS is inferior because it was made at a time when sending data over the GSM network was expensive thus it had to compress the picture so much as to make it lose detail. Now on high resolution screens such as the one found on the iPhone, and incresingly on other touch screen based phones, the pictures from MMS messages would look so horrid that it probably wouldn't help at all. Case in point, my friend just sent me an MMS of her holiday to the beach but I couldn't even make out if she was smiling or frowning because the picture quality was so horrible. If it were the full 2MP's and sent via email, I would've probably been able to make it out better.
I think a more pertinent complain to the iPhone would be the inability to send multiple pictures in one email.