I think we need more research on this. I found that it's not working well on every platform (mac)
Actually, what I was trying to do is reversely porting new label system into Cocos2d-x 2.x and use it for CocosBuilder extension to make my games cooler.
I finished this work and then found several problems positioning bug with TTF label.
Positioning problem with new label system can be easily examined if we create old/new TTF label together with bounding box