This would be interesting for the blue / green bubble debate. When you send an emoji, you send an MMS...
For instance, SMS seems to be preferred for online selling platforms in which only a phone number is exchanged. It's strange to add a stranger to your contacts for a one-time transaction, so I've often been contacted by SMS or email.
It might also help that it's increasingly common to pay over the TWINT mobile payment system created by Swiss banks for which mobile numbers remain the primary identification method for private users, not to mention SMS messages used for transaction confirmations or login codes. Given the default option of a phone number for many of these private sales, an SMS is probably more straightforward than adding a new contact to use another app.
As an aside, Unicode SMS messages fit far fewer characters per message (around 70 as opposed to 160), so most mobile devices still default to 7-bit ASCII unless the message can’t be represented as such.
I accidentally configured Windows Phone to use ASCII instead of UTF-8 for SMS a while back then the first time I tried to send emoji the phone chucked a wobbly and told me it wouldn't send due to a character set error. Once that was fixed everything worked fine, and I don't get the indication I'm sending MMS.