Don't dust any of that shit man. Just because cards "aren't used" by spikes doesn't mean you can't use them yourself. Spike decks aka top tier net decks suck to play against, so be part of the solution, not the problem. Have a card that doesn't see play? Give it a home and see how it turns out. Sure you'll lose slightly more, but it's way more fun than playing the same matches over and over and over again. Here's another thing you'll have to deal with when dusting your entire collection to craft spike decks: MIRROR MATCHES. Everyone knows how fun mirrors are right? BUILD YOUR OWN UNIQUE DECKS AND HAVE ORIGINALITY.
Plus you only get 1/4 of your value back which is laughable. Its like trading into gamestop