Here is my 2bar, but as a warning it will probably be an unpopular opinion.
1. Yes Rescue is a great course (if done correctly) and students can be greatly challenged and they can come away not only learning new skills, but learn something about themselves and their capabilities (often with ego bruised at some point)
2. When to take the course: When you are comfortable at the surface "self rescuing" yourself. Those divers who become flustered dealing with their own issues at the surface, who simply for instance can manage to climb out of their own gear without assistance, perhaps can't find a dropped reg, or deal with an inflator hose being disconnected or any other of the basic skills really aren't ready for Rescue. As with any course the basics from previous courses need to be practiced and consolidated prior to moving forward.
3 Rescue skills like all other skills degrade rapidly with lack of practice. While its not generally possible for people to physically practice the skills, they can mentally run through all the steps. I see this far too often with DM candidates, who might have taken Rescue within the last year, but look blankly when asked to run through "Unresponsive Diver at the Surface for the First time" (Sadly the same applied to Instructor Candidates - but I'm allowed to be less polite with them!) If that person can't manage in a confined water zero stress situation., how will they manage in a stressful OW event?
Someone holding a Rescue card who hasn't kept up to date and mentally prepared, is simply and OW student with a different card
Any diver who uses Rescue as bragging rights is a fool.
As for "Policing" other divers - it's just not possible, and perhaps intervention to another diver may be considered unwelcome - especially if they have more dives than you and (Their ego) thinks they're a more experienced diver.
As an Instructor, if I'm on a fun dive, yes my instinct it to keep an eye out, and yes I will intervene either underwater, or on the surface If I think it's necessary/worthwhile - Yes I've had quiet words with DM's too - But its not possible to police the whole group, and there is a line between caring for other people and enjoying a dive myself
Lots of divers are both selfish in that they concentrate on themselves only, but this is often compounded by the fact, these divers don't have the mental bandwidth to concentrate on anything other than the mechanics of their own diving, despite what their egos think, they haven't progressed to the point where breathing, buoyancy depth control etc are all automatic and require zero thought, allowing them to keep view the bigger picture.
The DM course while good if taught correctly, isn't a magic card. the quick 10 day courses are worthless. It takes a great deal of practice to become a competent DM when guiding, to be able to keep a mental picture of everyone's gas levels, to herd carts, and anticipate problems. People learn more guiding real customers than basic controlled scenarios. Trust me customers are unpredictable, and sometimes its impossible to control the uncontrollable.
As for protecting yourself from possible litigation, Conduct yourself within the limits of your training. Unfortunately we live in an era where people refuse to accept personal responsibility for their actions, demanding that someone else must always be at fault.