Actually, a cell phone user CAN send an MMS to you by using their MMS app and sending it to your email address. All they have to do is send the MMS to 121 (for AT&T users; I don't know the number for verizon users) and have your email address be the first line of text. Then you'll receive it as an email even though they sent it as an MMS.
For example, when my daughter wants to send me an MMS, she sends it addressed to 121, and the first line of text is my real email address (NOT my phone#@mms.att.net).
Not exactly elegant, but you do receive the picture in viewable form on your phone without having to log in to a website and enter the login and passcode. And when you reply, it goes to their phone since the from address is the email address of the sender's phone.