I sort of have the same debate. I am currently on the last step of the Legendary Firelands quest chain. The staff I have now is not a legendary, but just the epic Runestaff of Nordrassil, http://www.wowhead.com/item=71085
So i can transmog that. When i hand it in, i get the legendary Dragonwrath, but that one i cannot transmog anymore. So I cant hand in the quest.