It largely depends upon your alternatives. By taking the MSD, will your diving experiences be greatly improved, when compared with what you can learn with other divers? As your potential of learning largely depends on the Instructor on any training program, is it of value? The MSD card may be meaningless, but the experience you may gain isn't.
As previously noted, Divemaster is a leadership level. Because you can do a thing doesn't mean you should do a thing. Do you want to be a Divemaster or Instructor? If not, don't bother.
If you do, what type of DM do you want to be? Not all DMs are competent and many could improve their skill-sets by gaining diving experience. Becoming a DM doesn't mean anything unless you're competent. I recommend that you worry about your personal competence before you put yourself in a situation of being responsible for someone else's. When your good to go and you want to, move into leadership.
If he already has 3 specialties, what is he going to gain from getting the MSD card? The card is rescue and 5 specialties, right? How is he going to gain more experience from a card? OTOH I agree with your sentiments about DM.