Having drop-chances equal across all the classes is unfair on at least two levels. First, tanks, healers, and casters share loot while DRG/MNK/BRD don't so, on average, you'll get enough loot to gear up two BRD/DRG/MNKs in the time it takes to gear up one tank/healer/caster. A fairer loot distribution would have a tank/heal/caster/brd/drg/mnk distribution of 22/22/22/11/11/11%.
Even then, though--as the OP pointed out--the game is largely designed around groups with two tanks and two heals so you could imagine an entirely equitable loot distribution looking something like 25/25/20/10/10/10%.

I think either of those systems would be a huge improvement over the current--every piece of loot has an equal drop chance--system.