IMHO It's marketing. Even when they print a number, if it's like 2500, just exactly how "limited" IS that?
20 issued, now that's cool in and of itself.
But who cares? A lot of them are good looking, and/or fit other areas of our collecting interest.
So the number of them out there doesn't make them any less interesting in your collection.
Re-Sale value could be a problem, if there are so many of them out there that you can get one on eBay at any time.