come on

[Nezerius]

My boyfriend and I got into an argument about this very thing. My boyfriend's stance was "well, black mages are very characteristic looking; black clothing; black pointy hat, etc. You don't want people getting you confused with, say, a white mage or a dragoon, so they make it so you look like a black mage".. I walked about 10 feet and looked at a WHM who was wearing exactly what he said: Black robes, and a black pointy hat and I asked him "what do you think she is", his answer "black mage, obviously". Wrong.

So I agree, the restrictions don't make a lot of sense.

On the other hand, the first white mage AF robe (Healer's/Cleric's Robe) is pretty iconic to the series, and would be pretty difficult to make a close replica of, on a non-WHM job.
To me, a DRK running around in a Healer's/Cleric's Robe would be sillier than the same DRK running around in a bikini or a chocobo suit.