From the perspective of recently completing my DM and also starting to get into technical diving, I'd say that becoming a DM does improve your skill level in a number of ways. I'm thinking not so much of day to day stuff like bouyancy control (that's where tech training can really help) but in things like diver assessment, observation, water positioning, rescue skills. The teaching and leadership roles are also important. And by the end of the internship, my air consumption was way better.I always ask "why?" when someone says they want to become a DM/Instructor. As Walter touches on, so many people think it's a logical progression in becoming a better diver. It isn't. The route to that lies in technical training.
I also found it tided up a few skill areas where I'd gotten sloppy - if the DM isn't doing it right, the students won't either. There will also be a difference between doing DM as a course or as an internship, as to which stretches you more. IMHO internship is the way to go.
Last November a group of us did he IANTD Deep Air and Advanced Nitrox classes. Most of us were OWSI or DM and they gave us a workout. Everyone was stretched somewhat. We did most of the dives in the pitch black murk of the petrified pecan forest 140' down in Lake Travis. Definitely nailed all of us for skills.
So I think there are two strands - tech is great for individual and buddy skills, DM is great for overall diving and leadership skills, so both have their place.
