How much financial benefit you get from being a DM depends on the policies of your shop. Some give free air and substantial discounts on equipment; others may only give you free air when you are working with classes, and offer pretty minimal discounts. DMs can do some limited teaching activities, like Discover Scuba sessions in the pool (once signed off to do so), refreshers, and Discover Local Diving sessions, and sometimes you can earn a little money doing that. To my knowledge, in the US, you are not going to come anywhere near earning a living as a DM, or even as an instructor, so you can cross that off the list.
The other two benefits that are usually discussed about doing a DM class are improving your own skills, and enjoying helping with students.
The latter is definitely a benefit, if you like doing it. I really enjoy the pool sessions with OW students, and the dry suit orientation sessions I did a week ago were a blast. I enjoy DMing AOW and specialty classes, but I'm kind of ambivalent about OW. In our area, the water is cold and the viz is usually low, and the stress the students experience with the transition from the freedom and ease in the pool to the challenging open water conditions is hard on them, and hard on me. It is fun, though, when they come up with big grins, and can't stop saying thank you!
As far as improving your own skills goes, the only one I think DM really improves is situational awareness. A good DM needs eyes in the back of his head, preferably on stalks . . . you have to be able to manage your own diving and your own buoyancy while watching everyone in the water with you, and rapidly intervening when needed. If you can't imagine being able to swim 20 feet up in the water column, grab a student, vent their BC and get everybody back to the group in good order, then you're going to learn some things in a DM class, or acting as a DM.
The value of "demonstration quality skills" is, I think, pretty dubious. Being able to clear my mask in slow motion didn't make me a better diver. Being able to do it on my knees would CERTAINLY not have made me a better diver. The slow and exaggerated methods of demonstrating are very useful for demonstration, but they aren't real world skills, and unless you are asked to demonstrate them while maintaining your depth and position in the water column, they aren't going to improve your own diving at all. There IS some academic material in the DM class that I would guess most recreational divers haven't really studied, and I did learn some things by reading the entire PADI Encyclopedia . . . but you can buy the book without the class, and read it yourself.
So I guess I would say that a DM class is useful if you want to work with students or do guiding. You are unlikely to make back your investment in it, within the US, and you may not cover the expenses of your insurance. (I sure don't!) It is unlikely to make a significant difference in your own diving, unless your instructor is an unusual one.