Haven't legendaries in the past just been good for one tier, more or less? They are cool, but then you move on. I suspect it's good for the sha-touched weapon - it's be a little dull, if we did just keep the same weapon always.
However, I did enjoy the Lord of the Rings Online "legendary weapon" system. It was not perfect, but had some fun stuff about customising it with specific "talent" type abilities. As well as a lot of grindy stuff to level it.