I don't see how using Diurnal when you're paired with a White Mage is bad at all, in fact, it's a completely different set of circumstances. Nocturnal completely cancels Adloquium, which is awful because Adloquium is so much stronger. Diurnal has absolutely no effect on a White Mage's skills and the Regen even stacks. Besides, MP management is so much easier when you're using Diurnal just because you don't have to heal as often, while in Nocturnal those shields shatter within the next move or so.