String, from my experience the role of a dm varies from dive to dive and from location to location.
A good dm will know the spots to take divers too that you would not find on a site you hadn't dived before. To give you an example, I could take a diver on most sites when i worked on Koh Tao to show them where a scorpionfish was, rather than they find hem without help and accidentally put their hand on one.. ouch!
Alternatively, I hear time after time of people going to koh tao and not seeing any sharks - chumpon pinnacle for example is a site where if you dive in the right area you are GUARANTEED to see bull and grey reef sharks, but it's a question of knowing where to go.
Other dives might include a series of swim throughs, difficult to find if you dont know the site.
Given that you are an experienced diver I wouldn't suggest that someone like you would need to 'be watched' or told what to do, other divers though need to be, esp if you hve different groups on a dive boat - it can blow a schedule to hell if divers turn up late and mean that other divers miss the 2nd dive in the morning for example.
Greg Jenkins