IMO, the add-on value is the repository of decks that exist (and may or may not be free).

So an app-store of sorts.

As others have said, there are some provisions in place that make it allegedly harder to do a hard landgrab and keep people from freely sharing decks, to to me, even if it were so, I would not be too concerned.

In my opinion, the very act of creating one's deck is a key part of the learning. Maybe it's different for larning vocabulary, but as you said, it will be very hard to make those hard to share.

Learning a deck generated by someone else has never been as effective with me, so I think it's a false sense of time saving to use those.