I think Mustard Dave had a very good point:
there is a good chance you will a) have a load of bad habits to unlearn, or b) be a very competent diver who will get nothing out of the course.
I did very few dives between OW and AOW and glad I did so. I'd rather spend my formative dives with instructors and/or DMs to ensure I don't pick up bad habits and to learn new things early so that I can incorporate those items in to my diving mindset. There are things that I have learned that I would have been unlikely to find myself which are useful and some even essential IMHO that my OW instructor missed or neglected to teach.
I am aiming to complete my RD at some point in 2015 (I don't get to dive often enough to do it now). After that, I'm not sure if I will progress to DM or happily stay at RD.
I trained with PADI up to Rescue Diver and have the drysuit, deep and nitrox specialities. I probably wouldn't have bothered with the drysuit in hindsight - I got it because I thought I'd have to hire a suit, (which is difficult unless you can demonstrate competence), however, I bought my own straight after the OW course which I did in a drysuit. I want to dive less explored or even unexplored wrecks so I chose to do IANTD Advanced Nitrox and CCR.
PADI are a business, as are their affiliated instructors and dive centres and will gladly take your money. Some dive centres will try the hard sell more than others. Rescue diver is a good point to get to, but once at that stage, stop and think about what you want. Many dive centres push the DM course on Rescue Divers, even if they have no plans to go pro. Many are pushed through the system as soon as they hit 40 dives and convinced being a PADI-Pro is the pinnacle of diving.
Get to Rescue Diver and do loads of diving - join a club if you can. See what sort of diving you enjoy and decide what route you want to go down. I don't see the point in collecting all the specialities as you can learn all the skills by diving with experienced divers. The deep and nitrox specialities are a bit of an exception as the first allows you to obtain nitrox and use it safely and the second extends the depth you can dive to.
I won't say the DM course teaches you nothing useful - it will polish your skills, but it doesn't qualify you to do anything a Rescue Diver can do other than assist instructors and teach some PADI programmes. Something like an advanced nitrox course or GUE Fundies may be a better option.
If you don't want a career in diving, it can still get you some perks such as free diving if you help out at a local dive centre. For example, I know a DM who retired in his early 50s and moved to Malta. He's gone to a dive centre and told them he'll gladly take holidaying divers on guided dives. He told them he doesn't want paying, but he wants his tanks filling and his kit keeping at their premises. He point-blank refuses to hump kit, pump cylinders or get there at stupid o'clock in the morning to get kit ready for customers. In return he always has somebody to dive with and he gets free transport to the site, and sometimes a space on a boat - this arrangement suits both him and the owner of the dive centre.
Many PADI Professionals are exploited though, so if you do choose to go down the DM route, make sure you aren't being talked into it because they just want to sell another course, or add you to the pool of inexpensive / slave labour.