I dont get their purpose anymore. Once you ding max level everething is purple.
in hearthstone for instance, finding an epic card makes you excited, finding a legendary makes you go OMFG and wait with anticipation to see what card you've got.
But in WoW everething is the same collour they might as well remove it.
Imo all bosses should drop blue items and green should be from trash. Purple items should be from the last boss in the instance and they should be rare drops. It would add to the excitment when you finally get one. Otherwise there is no point in having the system.