If you ask me I think we should reserve our judgments. What, after all, is wrong with a Christian wearing a Christian symbol? For any reason? Ultimitaly it will be to glorify God, whether or not they truely understand what it is they are wearing.
More over, perhaps for these baptist their introduction to the Holy Mother (and a new perspective on Christ through her, which is the whole point of the Rosary) will be through, first wearing one as a statement. Perhaps later on, they will become more curios about it, discover how scriptually based it is, and start actually praying it. Such a thing can not be bad.