User blog comment:IamJakuhoRaikoben/Chapter 384: Attack of the Stars, Review./@comment-3424593-20140516074407/@comment-7418318-20140516085331

That depends on how the key is restored. If the Spirit King just snips his finger and the key is whole again you are absolutely right.

But if the restoration of the key comes with a sacrifice of it's own, it'd be a different matter. Maybe something like sacrificing a limb like in FMA. While I can see Lucy doing it to restore Aquarius' key, it'd still leave summoning the CSK as a last ditch action only done in the most dire of situations.