I am wondering what (if any) advantages ND has over CP. I can understand if you were using a CP and it could not slow the shutter speed down enough one might need to use an ND4 or ND6 or more.
But for general use, is there any advantage of using something like an ND2 over a CP? Perhaps the more appropriate question is, in what situations would an ND2 be more appropriate than a CP or vice versa?