Ya I agree that Red Mage sucks as a sub. Beyond fast cast there's no point.

MP? Sub as Black Mage or White Mage they give you more then a Red Mage plus you get the auto refreshes as Black Mage or a auto regen as a White Mage.

HP? Warrior and Monk get a hell of a lot more HP then any Red Mage, plus they get combat skills and strength while your at it.

I think in the end it's sort of clear that a class meant to be good at everything, sucks as a sub. Subs are meant to add to your current class skills and stats, and also be specilized. I mean a Warrior who subs Black Mage for the MP is just a idiot. If you want HP sub something that gives the best HP. You want MP sub something that gives the best MP. Need STR sub something that gives the best STR. But don't sub something that will make you averagely weak.