In the US most people are still on a monthly plan that includes "unlimited texts" (inc. MMS). In many other places a single MMS can cost you between 5c-$1, so people use WhatsApp, Google+, or many other free (or cheaper) alternatives.
But MMS is definitely popular in certain regions. It is also very unpopular in regions. If you're in the UK you could very easily disable MMS, but in the US? I wouldn't...
I thought phones didn't even support this anymore. I've never seen anybody using it, and I wouldn't know how to send one.
All the multimedia messages I see people using are transmitted through the Internet.