A warrior tank who doesn't have trouble tanking Naxx will have trouble tanking non heroic level 70 instances... you'll have no rage. However, if you want the practice then you should definitely go for it! I think many of us run into the problem of wanting 5 toons for PvP while trying to cycle in a tank. Personally, I think the bests option may be using a druid tank and switching to moonkin for PvP (or be a 50k HP bear swiping everything they see while your shamans toast anything that gets too close)