If you start an SMS (MMS?) group chat with >10 people, any AT&T customers (and possibly others—I'm not sure) attempting to reply will end up replying to 10 arbitrary members, leading to a huge pile of separate chats, each missing a few people. Even under 10 people, SMS/MMS group chats don't support features like adding/removing group members, among others. It's a substantially worse UX.