You're not entirely wrong, but keep in mind you could save shadow crystals by just remembering what your decks are or writing them down. The deck slots are a convenience purchase meant to make it easier to keep multiple decks for each hero without the need of an outside or third-party assistance. If you're on the go, or are in a spot where you can't really access your deck editor (maybe its on a different device or something, idk) or just can't be bothered, you can use those for a small amount of SC (so small you can level your hero up to get them all fairly quickly).
Also, thanks for the suggestions everyone.
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