I don't really understand why anyone would want to be a DM except as an intermediate step towards becoming an instructor. If one's goal is to teach, then go for the DM cert and move on to instructor as soon as possible.
@SammyW
Lorenzoid's post above was pretty much the way I thought before I took the PADI DM course.
A lot of people say that the course doesn't make you a better diver, or improve your skills, rather it is just a step towards instructor. I have to disagree (the caveat here is it depends on your instructors)
Using myself as an example:
Prior to starting my DM course, I was already a BSAC Dive leader (equivalent cert), I was qualified to 50m and mixed gas deco. I was pretty happy with my buoyancy being able to hold a stop with a total up and down deviation of less than 12", and my general diving was done with groups of very experienced friends in challenging (big current) areas.
So entering DM I was pretty confident that everything apart from the swim tests would be a breeze. However I was on a self imposed "gap year" from work and thought it would be fun and I'd probable learn something. I certainly did.
Buoyancy - as above My skills were pretty good, however holding a stop at 5m and below is hugely different from keeping neutral in a shallow pool (1.5m) while slowly demonstrating skills. My fine tuning of breathing improved remarkably and I learnt to slow movements down even further than they were already. I also re learnt skills I'd long ago forgotten.
Finning. Again a higher level of precision is required to hold position with a student in confined water where you are concentrating on teh student rather than your finning. Using your hands (not that I did) is not role model behaviour. I certainly got to fine tune and improve my precision finning.
In my normal divign I was with experienced people that you didn't need to worry about. Guiding strangers teaches you another level of concentration. It's mentally challenging because you're thinking for the group rather than just yourself. You'll learn to have eyes in the back of your head etc.
While I'd never dare to gas check my wife, according to her post DM I was a more attentive diver.
And yes if you are thinking about teaching, then DM will let you know if you enjoy it or not..
In short, personally I believe there is value in the DM course, everyone can learn something - a real world example is a very experienced and highly respected full cave instructor on here who has chosen to undertake GUE Fundies, because he recognises that there is always something to learn.
Anyway just my 2bar..