I was taught to use 6 minute markers during my own PPL training. In my experiance on my own navs, they are more of a hinderance than usefull. For a start, they are only usefull if your ACTUAL ground speed matches your PLANNED ground speed. If your actual ground speed is more than a few knots off plannes ground speed, each sucessive 6 minute marker becomes more and more in accurate(and confusing). I know I spent more time trying to red the markers in flight to match the actual ground speed, rather than flying the plane.
After 3 navs, I abandoned that method in favour of 10 or 20 mile markers(just count the number of markers off the map and you have a good estimate of distance).
For more advanced students I teach the method of picking a single identiable on track ground feature(between 1/3 and 2/3 of the way along track) for a groundspeed check.
Greatly reduced workload is the advantage.