DM cert gives me buckets of practice on the basic skills, helping in classes, things that I know my buddies don't have -- we may all have the best of intentions, but regular practice of certain basic skills just doesn't seem to happen all that often with most people. Run through all of the skills every couple of weeks and you will be good at them. Work with a lot of people on buoyancy and you will be conscious of it, and also get to the point where your own buoyancy control really becomes good.
DM work ends up making you a much more aware diver, keeping an eye on others, averting problems before they arise and learning to cope with them. There's a regular dive site that I go to with a depth change that catches new divers off guard, a tendency towards an ascent. Through lots of practice you become aware of a buddy hitting a buoyancy problem, can stop it before it goes west, without thinking; my regular non-pro buddy, quite experienced and safety oriented, just doesn't have that reflex even when pre-briefed and diving with someone new.
DM cert does have an annual fee, to remain active, but then again it also includes membership in the PADI DIving Society -- get their mag, another pro-oriented mag, some discount offers.
DM cert does have insurance to remain in status, but then again I get that through my LDS, at pretty low cost. As a DM I get free air fills, plus healthy equipment and additional training discounts, plus shop credits based on the number of students in classes that I help out with during dives.
An active DM, working with an LDS, can also qualify for key man discounts; policy depends on the shop one works with of course. I just got a new quite high end computer for a huge discount, several hundred less than even the best online pricing. My savings on that one item more than covered my annual fee and insurance costs, totally ignoring the discounts I qualify for on all other equipment and the shop credits.
On vacations, pros can be given more leeway by dive ops, less "thou shalt be in a herd on exactly this route with your buddy". It's a professional courtesy -- knowing that the pro has had more training on risk assessment, has had to do certain things related to self-reliance.
So, end of the day I get lots of practice on the fundamental things, vastly better situational awareness, and come out ahead financially -- I don't make "real" money, but it pays for my dive toys, and neither am I taking on the additional responsibility (and headaches) of being an Instructor.