The fragmented and quirky MMS implementations in the wild render MMS functionally useless, especially compared to what feature set an app can have. I've seen MMS implementations that send replies to only the sender of the original (so some replies, from better implementations, end up in the MMS group, and some end up only sent back to the sender, resulting in confusion); I've seen MMS implementations that allow you to "like" a message, and this is implemented by just sending "I liked this." as a message back to the other clients — which can't interpret it as anything other than just a normal message — resulting in confusion.
Did you know that MMS can transmit slideshows[1]? I didn't, until my father somehow sent me one. The UI that Android has for that is — naturally — a complete afterthought. (No way to pause the slideshow, no way to navigate the slides, nothing. Just one run through the animation at Warp 8.)
[1]: https://en.wikipedia.org/wiki/Multimedia_Messaging_Service
