Why can't I receive group iMessages when my iMessage is turned off?

I currently don't have iMessage active on any of my devices. I can receive text messages from other iPhone users that use iMessage but I cannot receive group iMessages. I have activated iMessage on my phone and iMac to see if the unreceived messages would come through but they appear to be in no mans land. Does anyone know how i can retrieve these unreceived messages? And how can I receive group iMessages without iMessage being activated?