I tend to have a higher level of respect for someone that is a "Dive Master" then a "Master Scuba diver".
MSD rating is essentially an extra card, it is not a professional qualification and isn't worth the plastic that states it. This isn't to say that a diver with one is useless or unexperienced, but simply they just have an MSD card.
A dive master, is a professional rating and for someone to earn it they must have learnt and demonstrated a set of skills that make them worthy enough to be employed, not just as a diver but also as someone that can lead and organise a group of divers and keep them safe.
This is essentially the same for instructor, the only difference is, that students aren't qualified in the field of diving (or the sub specialty they may be doing) and thus the instructor must also be able to demonstrate relevant skills required to teach them and be able to keep them safe.
As a Dive Master in training, I have spent a large aspect of my training learning when someone really is a master at diving or whether they just have a fancy card that says they can scuba dive.