That can work if groups of contacts are still grouped by app, like friends and family using different apps and you don't need to talk to both at once. But in the example given by the OP, if one friend wants to use a different app than other friends, it would be really awkward. You'd have to copy every message to your friends twice -- once on the app with the other friends and once in the app for the contrarian friend. In practice, that friend would be left out of many social events.