Barry,
This is just my opinion but in order to get any lasting benefit out of the DM course in terms of your development as a diver you need to teach.
The course will give you some new theory and it will flesh out a few things that you already know, but if you're on top of your dive theory already you'll find that part of the course pretty straight forward.
The remainder of the course isn't really a diving course, although you'll need to make some dives for the underwater map and some of the modules. My buddy and I mapped out the reef balls at Dreischor for our mapping project and that took us about 10 hours under water (in the middle of the winter). What the course does is prepare you for assisting with the various PADI programs. You'll learn things like how to demonstrate a mask clear, but it's understood that you can already clear your mask.
However, having said that, *if* you decide to teach afterword, then the constant repetition of the basic skills will solidify them in a way that normal diving doesn't. In that sense, teaching has given me a feeling of control over the basic skills that I never had before. It probably does this for a lot of active DM's and instructors.
The same things goes for assisting in open water but, in my opinion, to a lesser extent. In Holland you'll find yourself much more in the babysitting and "street sweeper" roll when assisting in open water and when doing guided dives you'll be mostly busy with the logistics. In fact, when I do BD's (begeleide duiken) I usually arrange enough DM's to supervise all of the divers who need it and I spend most, if not all, of my time on the process. In fact, if I can manage it I try to stay top side to keep things organized.
Personally I dislike BD's. I DM mostly OW, intros and scuba reviews and it takes me to the pool one evening a week and every now and again in open water. Most of the time I avoid guided diving because I feel I get more out of diving when I can do my own thing. (It's also more or less volunteer work in my case so I feel completely justified in looking for a balance in this...)
of course, you'll need to get some experience with it to see for yourself what parts of the job you like. i know guys who only do BD's because they hate the pool work...
In terms of costs you'll get set back about 190,- per year for membership and insurance after you're certified.
If you're interested in seeing how the poolwork goes then you can meet me in either Rotterdam or Amstelveen and tag along to watch a scuba review and then we'll have more a chance to chat about it if you have any questions.
R..