Hi Sue(M),
I'm not sure what you mean by undesirable features in themed weights. The Christmas weights from Whitefriars were all based on Christmas topics (some religious, some not). The last two, of the Partridge in a Pear Tree and the Christmas Bell, were viewed as excellent technical examples and desirable in their own right as well as being scarce. This boosts their market value as does the fact that many collectors would buy the others and then want to have the complete set, thereby increasing the scramble for the scarcer ones.
Cheers,