Hearthpwn

Iv been playing since gvg came out and i was wandering if you guys keep your extra copy of legendarys for a possible feature in the future like trading. As up to now i have dusted my duplicate legendarys 

Possible nerfs are reasonable but other than that not really.  My collection is pretty good so I have no reason to dust things right now.  If you don't need the dust just keep them.

If you look at the history of Hearthstone, very few Legendaries have been nerfed since Classic. Except for very specific circumstances, I wouldn't imagine any being changed in the future.

There's also the chance that some cards may be moved from Classic to Wild.  To be safe, don't dust any Classic cards until the next set comes out because by then there will be nerfs or cards moved and they will primarily be from Classic.

I never Keep them and i don't think that there will ever be the Change of trading them.

I guess it's risk vs reward. If you need the dust now for a specific purpose, a card now is more valuable than MAYBE getting full dust later. If you don't need the dust, then there's no harm holding on to them until either you need a card (see the first point again) or it gets nerfed. 

Whatever you do, don't hold on to them for trading. Blizzard don't want trading because they don't want new players ripped off, and crafting is specifically in the game because trading isn't.