I think you are putting a bit of a narrow rep range for what is useful. I know different muscle groups tend to respond better to different rep ranges and 6 is a bit low. I like 5-12 range myself depending on what I am doing and how intense I want to go. Also try 20 rep squats sometime…put on a weight you know you can get a solid 12 times and keep going until you get to 20 reps pausing at the top…this has been proven to be one of the best techniques to add size to your legs. Also going heavy all the time isn’t the best thing to do, there are times you need to change it up and run a light cycle for like 3 weeks or so to keep progressing.
But I do think people who put a light weight on the bar and rep for like 15-20 reps while not really straining themselves are for the most part wasting time (depending on their situation of course)