Thanks The Replay - Classic and Core are separate formats now. I think a lot of the confusion comes from the old set being called Classic - and some of those cards have now appeared in ‘Core’. But you cannot dust the ‘Core’ copies of these Classic cards. Only the ‘Classic’ copies of Classic cards.
The only benefit to keeping the ‘Classic’ copies is if you want to play ‘Classic Mode’ (as Core cards are not allowed). It is basically a Hearthstone reset - no expansion cards allowed. Some people seem to like it - I think it’s just Blizzard trying to cash in on new players - forcing them to craft/buy ‘Classic Packs’.
The other reason to keep them could be because of future nerfs - for which you would then get a full dust refund (like with the ‘Classic’ version of Ysera at the moment). That said - sitting on the entire ‘Classic’ Set on the off chance you might get a nerf - is a bit like having all your money tied up in a 0% interest account, in the hope that one day the interest rates go up and you can make something on your collection.
I agree though - I think it was a really underhanded move by Blizzard to make the entire set worthless in Standard play overnight and offer no kind of apology or refund. I don’t want to play ‘Classic Hearthstone’ and as you say - everybody got the Core Set - so it doesn’t count as any kind of compensation.
There is a full thread where we have hashed out all of these arguments: -