The Rescue Diver program is not designed to turn people into professional search and recovery divers, just as the EFR (or any first aid course) is not designed to train people how to be Paramedics. It's a collection of basic techniques to give the average recreational diver some information and techniques on how to deal with emergency situations - it doesn't mean a certified rescue diver is expected to deal with any incident that might occur, but it's a good starting point, and at the very least it let's people know that there are possibilities available should something occur.
In many locations then yes, there is going to be an experienced DM or Instructor available to deal with dive accidents, but there are also many locations where this is most assuredly not the case. This might be because the DM or instructor is relatively new or not confident to deal with these situations, it might be that they are poorly trained themselves, and it might happen that there are no certified dive professionals in the group. In resort locations then yes, the dive operator is usually going to insist on a guide, but there are plenty of dive spots where two relatively inexperienced divers can buddy up and head into the water. It is also true to say that in some locations - leaning towards the developing world here - you can often find local guides that are not certified DMs because the program is unavailable in their language - and there is no law in these places that says you have to have a professional certification to be in charge of a dive group.
It is possible to rescue people using the basic training - I've moved on a bit since my own rescue course but I haven't had any further training in diver rescue other than what I've read about. As an instructor I have had to deal with some emergencies, and my colleagues and I did exactly what it said in the Rescue and EFR training. It's highly likely that we saved lives as a result of that. I highly recommend the course for exactly those reasons and my personal philosophy has always been: "Rescue courses are not designed to turn people into professional search and recovery divers, they are there to give lay people a basic introduction to techniques that may help to save lives in the event of an emergency. If Rescue training only ever saved one life, then it was worth it."
Cheers, safe diving,
Crowley