Not exactly sure if there was a change in MoP that allowed this or not, so im not sure if its intended or not.
Delete this post if this is intended or a repost.
Anyway, Many times, I was able to level up my herbalism by picking strictly Grey leveled herbs. And this isnt a rare Occurrence either. I gain levels from grey herbs almost every other herb I pick.
Green-Tea Leaf is grey to me yet it still levels me up.
Just thought id share, cause as far as I remember, grey herbs, ore, etc never gave you levels anymore.