I don't agree at all about Luther as a first name. In fact, I rather like it. What about Luther Vandross, just to name one example. To me it has musical connotations and reminds me of a violin maker.
I don't mind how Luther sounds, it just reminds me way too much of MartinLuther, with all the implications that as, or possibly of MartinLutherKing. And if one wants to honour Dr King's memory, I think Martin would be better.