I always found building up Gau's rages tedious and boring, especially later in the game when you keep encountering enemies that you've already learned and not finding the ones you actually WANT to learn. And of course there is nothing that tells you exactly what abilities from whatever enemy you get. Also, in my experience there are always two of them in each rage, one of which is usually an average physical attack and as there is no way to control that, he is not terribly reliable. It is nearly impossible to fight strategically with Gau. The same can be said about Mog's dance ability but the difference there is that dance is optional, where rage is not. If rage were a secondary ability (such as dance, blitz, steal, etc) instead of the default battle command, it might have worked out better.