I'm planning to use this feature to let me know _where_ my cards are.
A card can be in one of a few locations:
(A) A named collection storage, such as "Trading Map", "junk drawer", "box UncU", etc.
(B) A deck.
Assuming that you can tag any card with any number of tags, then I can just make tags for all of my collections. I'd also have to make a tag for each deck, which is fine by be, and I'm not sure having an option to always have a tag for each deck is useful, because I usually have some decks registered that I haven't built yet and possibly even no further intention to build (ideas, precon decks, etc).
I'm not sure if this was what the original suggestion had in mind, but it looks like being able to create tags, and then tag each indvidual card, is going to take care of this feature request too :P
Keep up the fine work!