Ultimately you'll get most of your gear from emblems of heroism or crafted. Having gear versatility is nice, but really not something I concern myself with. I'd worry more about buff stacking (consider that switching a mage for a lock/DK/moonkin) would boost the remaining two mages dps by 10-13%. The main reason to go leather/mail classes is squishyness - in pve scenarios you wont be taking very much physical damage on your slaves, so there's really no incentive to spend item-points on AC (except for the fact that leather/mail-classes usually have a higher base stamina).