I don't see the point in doing the DM course unless it is a stepping stone to becoming an instructor. Although I agree you will learn more about diving, there's nothing you can't learn by just joining a club and diving (apart from the PADI indoctrination process).
I'll offer hree points that may influence you to take DM training as a stepping stone to tech even if you have no ambition to become an instructor.
First, DM's accumulate BULK time in the water and BULK experience sorting out SCUBA gear. If you take the DM cert, you will accumulate HOURS and HOURS of pool and ocean time during your internship. If you have a good instructor, you will very likely be able to execute all the basic skills to demonstration quality without thinking. And you will get to participate in more than one rescue class, usually acting as a victim. I can tell you that being the victim over and over for Rescue Divers in training will absolutely teach you what NOT to do. There is no substitute for BULK time in the water when honing your skills... so, there's that.
Somebody with 100+ dives, OW, AOW, RD, Deep and EANx should not need to become a dive centre's kit-bitch to learn to set up kit - you should know how to set up your kit by the end of OW. Also, the tech kit is different - why practice setting up a single tank and stab-jacket set-up when he is going to be setting up a twinset, wing, Hog-loop, deco-bottles etc?
Why learn to do the drills to demonstration quality? When he is doing tech training, he will be learning to donate a long hose while neutrally buoyant - not kneeling on the bottom and holding his arms up while somebody takes his octopus.
Time in the water can be gained by just getting out and diving, and in conditions that are a bit more challenging to those that students are usually taught in.
Second, Tech is equipment intensive. It's quite an expensive hobby, even for folks with some cash to burn. If you do your DM first, you become a dive Pro and may be able to take advantage of Keyman deals or your shop may help you out. You might even earn a bit of money toward your tech gear while getting some dives done.
Maybe so, but is the discount going to cover the costs of training? How much do DMs get paid? Where I am it's nothing. If money's tight, you'd be better off doing a bit of bar work in the evenings to finance kit.
Third... have you ever met a tech diver that you had a hard time connecting with because they were so... Techie? Divemaster training includes learning how to present yourself as a professional, a trusted advisor, a competent diver hopefully without any hint of condescension. Most of the divers I know are great folks anyway, but tech training is intensive. When you get immersed in the required discipline for Tech diving, it's natural to react to the things you see recreational divers doing. But a DM absolutely learns how to smile and lend a helping hand without making average divers feel self conscious. Do tech courses teach soft skills? I think it matters because we do not dive alone.
Potential Tech divers who have fewer than 100 dives under their belt might want to think about these factors. DM training can be a valuable stepping stone for future tech divers, even if they have no intention to become instructors.
I've met many tech divers, one ore two are knobs but then again, so are some recreational divers. In general though, I have never thought the majority of them need training on how to speak to people in a helpful and considerate manner. They are great skills to have but if an adult has to pay PADI to learn simple courtesy, there's something wrong.
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