Crafting Mode in Hearthstone offers the option to Mass Disenchant cards. The in game screen describes the option this way:
Disenchant your extra cards to create Arcane Dust
You can't have more than 2 copies of a card in your deck (or more than 1 legendary), so you might as well disenchant the extras and turn them into Arcane Dust!
Is there a reason to hold onto these extra cards? Turning them to dust is permanent and I'm curious as to why the game is giving me the option if the only use for them is to turn them to dust.