True.
However, if a phone is capable of receiving MMS and was made in the last 3 years or so, it can still send MMS from the MMS client to an email address. The phone will also have a dedicated MMS email address tied to it, so if you send an email to that address, it will be received by the person as an MMS and they won't know the difference. No email client or data plan required.
The only thing that changes is swapping a phone number for an email address. To make this even easier, all the person has to do is add the email address you have synced with your iPhone to your contact info in their phone. Now they should be given the choice of whether to send it to your phone number or your email every time they write you an MMS. You can make your life easier by doing the same thing and adding their MMS email address to their contact info in your phone. Boom, done, simple.
Yes, I know email isn't instant on the iPhone at the moment, but 15 minutes hardly kills me. If someone sends me an MMS, it is usually meant to be funny, and I can deal with not getting it right away. If someone needs me right then and there, they will text me or call me like normal, they won't MMS me.
<shrug>