Actually Red Mage get's fast as well. And in addition, unlike a Ninja, can cast Life and other cure spells. Anyone who knows how to use a Red Mage to thier fullest knows that you can't just take one aspect of them, like the other classes, and just do that one this over and over. A truly effective Red Mage will be nuking as needed, curing when needed or if the White Mage is taken down, and meleeing to help kill/damage enemies. Not to mention they add much more MP that can be put to better use than any Ninja, in addition to them learning more spells and wider variety of spells. In addition they can help get through the tough dungeons by helping the White Mages conserve thier MP for healing later on.

Over all I'ld say a Red Mage used right is very useful.