Well I know I am going to be playing both Rebels and Imperials, (which I am surprised to see so many people playing Imperial only) so I will report on my findings as time goes on.
But curious question, why do players only want to play one side? I understand the cost of keeping up both as the game gets bigger but right now until wave 2 it would be easy to play both without playing Keeping-up-with-the-Johnsons. I personally like the Art of War approach where if I can understand how both sides play personally I can be a better Admiral.