Normally it costs much more dust to create a card (4X or so, depending on rarity) than you get back for disenchanting it.
But occasionally, Blizzard nerfs (i.e. changes negatively) certain cards if they feel they have balance issues. When this happens they allow such cards to be disenchanted for the FULL dust value - a really good deal!
This has happened in the past with cards like Unleash the Hounds, Pyroblast, Leeroy Jenkins, and Gadgetzan Auctioneer.
That's why it can be better to hold duplicates until you actually need their dust to create a new card. There's a small chance that their dust value could go higher in the event of a nerf.
Thank for the clarification!