IMHO, the .500 Special was developed for folks who bought the .500 S&W, and soon discovered that it was more fun than they could stand. This is the reverse of the development philosophy for the .38 Spec/.357 Mag. and the .44 Spec/44 Mag..
The void that the .500 Special seeks to fill is already occupied by the .50 AE. I think that with a little forethought, Smith could have engineered the X frame .500s to accept a full moon clip of .50 AEs. Now THAT would have been a good idea ! Just my opinion.