Usually the Buy X Get X Free deal ends either when all retailers in the community have their stockrooms stacked to the ceiling with the junk or when the distributor goes out of business. Another possible cause is if the manufacturer itself goes out of business.
I think this tactic is mainly used to get away with making big bucks on sales of one case only, but I think that because so many people do it, it becomes almost necessary if your product is too similar to the ones that are sold that way.
For specialty shops it's unlikely that the X - X products will be a good thing to get; perhaps on rare occasion but yes I'd say these deals are often suspect.
Keep in mind that the King Of Pop\'s crown is not made from a bunch of melted aluminum cans!