Given they only featured in a small fraction of Final Crisis, and they could have been replaced by having ANY random GL under Granny Goodness' control, I doubt it was simply a case of Morrison wanting them there.
Johns clearly liked the idea of displaying that the Guardians were moving along the wrong path, post-SCW, and this is a great way of doing it, by showing they haven't learned from past mistakes. So they might have been Morrison's idea, but Johns went all the way and developed them.