[imranhou]

The primary limitation here is inability to receive responses back from users. From a business revenue generation angle, that's probably not enough of a feature set to monetize for the carriers.

[woodruffw]

I'm not sure whether it's universally supported, but AT&T at the least actually does support responses. I can text any email address from iMessage and receive an email via SMTP from [e-mail], and responding to that email also delivers correctly (and is even threaded).

Edit: One funny thing is that iMessage will detect gateway traffic from/to email addresses that are tied to iMessage accounts, and put those into a "blue bubble" chat instead of the normal SMS/MMS "green bubble" chat.

[dqv]

You can reply to these messages and it goes back to the email that sent them. At least on my carrier.

[alfredosequeida]

I can also verify that replies work (using T-mobile).

[nulbyte]

Not only can you reply, but you can compose new messages using an email address as a recipient.