And may be it was short-hand for a complete absence of character ....
Thinking back to a fascinating lecture David Doughan gave at Oxonmoot 06 about how Tolkien's attitude to women changed as he aged, I think Luthien was too much a product of his earlier attitude. Having heard reports of Priscilla being (unsuprisingly) formidable intellectually, I can't help speculating if being the father of a clever and strongminded daughter forced him to alter his opinions.
Oh that Romeo and Juliet stuff ... silly way to carry on Beren is no Kester Woodseaves - he really was "a man to die for".