I am not sure, but I believe the only difference for grey mobs is that they will not drop the rare itens.
I mean .. if you are in TS hunting scarecrows and it is at least green to you. You have a chance to lot a "treaty for Treasure" scroll for the instanced quest. But if this same scarecrow cons gray you won't have a chance at all to loot this scroll.
This is my understanding for grey mobs .. never expect getting good loots or XP for them.
Gray mobs can be used for quest, but I believe not all of them will give you the point for killing.
someone correct me if I am wrong.