Aetherkey Piece > Completed Aetherkey
If you want to do the achieve to open all 5 chests in the time limit, you’ll need 5 keys. Once you’ve done the achieve imo there’s no point unless you really want another piece or two of blue gear in your bags.
At least they seem to sell a bit now. For the longest time after getting that achieve I was deleting them for bag space.