I have to agree with Gudu here but with lots of others as well.
As a relatively new diver, I've never dove with a guide yet.
But a very reliable and trustworthy source told me this:
1) You are responsible for your own gear but if you are doing a guided dive, the divemaster(s) on board should be keeping an eye on the divers as they prep their gear. Don't LET the DM or DG prep your gear. (He told me some scary stories of DG's trying to prep people's gear and an accident occurred underwater that luckily just involved a tank coming out and noone getting hurt because he happened to be behind that person.)
2) A DG or DM should be showing you the good things on the dive.
3) If you are going on a boat dive, there should be a DM (not just a DG) on the boat.
4) A good DM will help with free flows and other bigger issues.
5) You and your buddy are responsible for everything else and your DM might be occupied so you better be able to deal with any bigger issues on your own!!
When we were in Bonaire this last time I had nasal polyps bursting so opted not to dive the last day because they were causing 3 hour headaches after dives. But I wanted the DH to still be able to dive so I suggested he sign up for 2 boat dives with Buddy (where we were staying). The 2nd boat dive only had 3 people signed up so I decided to go along and snorkle so that the boat could still go out. Irene and Gwen were AWESOME DMs. As they were gearing up, Irene was paying attention and noticed one gal hadn't hooked in her BCD hose and helped another guy with a free flow. My husband said that while on the dive she pointed out some things that there was no way he would have found on his own had we done that site by ourselves. She also "navigated" although they were allowed to stray a little bit. As I was preparing to go in to snorkle, Gwen told me a few places to look as I may see some great things if I was paying closer attention. I had a great snorkle and saw not only the anchor he was talking about (I wouldn't have spotted it on my own) but had 3 or 4 turtles come right up to me because he told me if I moved a lot slower in this particular area I would likely have a better snorkle. He was right.
That was my idea of what good DMs should be!
Oh, I picked "navigation" but I don't mean "always" and I also picked "manage major failures" but think it should be "HELP to manage major failures."