The market would be ALL android phones. Apple would never allow seamless interop. It's not in their interest, let alone opening themselves up for more exploit possibilities.
There does seem to be an in group / our group thing for teenagers and people in their 20s though.
The biggest feature benefit of group imessaging is sharing images/video at original quality. But I feel like that is also a benefit for WhatsApp, in that its compression results in quick messages.