There are very few dive operations that I use that I don't know the staff of, so most of them know I am an instructor before I even walk through the door. However, with a dive center I've never been to before when it comes down to showing them a c card I'll usually pull out my AOW. It usually comes out during the day that I am an instructor, but I just dont feel the need to scream it to the world. From personal experience instructors are either the best or worse diver you come across and I prefer to let people draw their own conclusions.
As for the OP question. I say why not to Master Scuba Diver. It's not the card itself it's the road you take to get there. rescue diver, deep spec, ppb, and even all the other ones have something very unique to offer you and once you complete those 5 specs and rescue course I like to think that you have a bit more of a grasp on safe diving practices and that is REALLY what that card is saying.