This may be true for responsible users who practice harm reduction, but it won't hold for habitual users or even occasional users taking 2-5x the recommended "s

This may be true for responsible users who practice harm reduction, but it won't hold for habitual users or even occasional users taking 2-5x the recommended "safe" amount, which is extremely common with MDMA. You could make similar arguments for almost any drug, most of them will result in worse and longer lasting side effects when they're used habitually or in high doses. I think GP's assertion is generally valid, most MDMA users aren't responsible and there's a lot of work to be done on the harm reduction side of things.