Most the DMs I know have done it for the wrong reasons. I used to be an active member of a dive club that is attached to the PADI outfit that taught me to dive. I regularly see them trying to talk Rescue Divers into taking the DM course, despite them having no intention of working as one. I experienced this myself, and eventually had to make it clear I am not doing it.
Most of the DMs I know are club members, who dive purely as a hobby. The amount of money charged where I am barely covers the costs of the course; all of the instructors do it in their spare time, and all have well-paid careers, so none are bothered about making money from diving. The instructors there get minimum wage (it actually works out less if we look at the hours they really do). The DMs do not get paid at all, again, they do it as a hobby.
The way I look at it is I would be paying for the privilege of working for free; I could say 'no', but the reality is that every time there is a course on, the owner is on the phone to every DM, asking them to help out. I see divers come through the system from novice to DM, and they barely seem to dive anywhere other than the training quarry.
Of course, others will have different experiences. One of the DMs that came through my local school was fortunate enough to retire early and move to Gozo, Malta. He works for a local dive centre as a dive guide. Maltese regulations only allow divers qualified to 30m to dive without an instructor, so he tends to only guide experienced divers. He doesn't get paid, but he has an understanding that he doesn't hump kit or fill cylinders. His reward is he gets regular dive buddies, free transport, and free air fills.
A Rescue Diver will become a slightly better diver by doing the DM course; your rescue skills should become more polished, as will the skill circuit. With the latter though, I wonder how often people perform the skills exactly how they are taught when they are needed for real, rather than face to face as they generally are in a practice situation. If you are looking to just further your skills, GUE Fundies is a good course to consider.
Dive Master, the first level of a DIVE PROFESSIONAL and as such you are held to a different standard of conduct/liability. You do know your DiveMaster card EXPIRES EVERY YEAR unlike your certification cards. Oh and lets not forget the INSURANCE you have to pay for which is not cheap...
These only apply if you are actively working as a DM. If somebody just does the course to further their own dive education, they are no more culpable in the case of an incident than any other diver. The DM qualification does not expire annually - you just become inactive when it does and cannot work
With regards to Insurance, it depends what you are offering. If you are working as an independent DM, you certainly need to have your own, but if you work through a shop, I'd expect to be included on their policy. This is the case at my local PADI school. I wonder how many employees in other jobs have to pay their own liability insurance? Over here it would be illegal to expect it.