Here's what I think is a valid question, or argument. There are two studies that show that a person can lift more with the hex bar, than with the straight bar. And there is ample empirical evidence that people can lift more with the hex bar than with the straight bar (check out
Strength Level - Weightlifting Calculator (Bench/Squat/Deadlift)). And try it out for yourself. Whatever is your max deadlift, grab a hex bar, load the plates and try for yourself. It's easier to lift those same weights using the hex bar.
With that in mind, we perform the low bar squat primarily because we can "maximize the use of all the muscle that can potentially be brought into the exercise and thus be strengthened by it". I know there are secondary reasons for low bar (less stress on knees - for example), but the primary reason is that it engages more muscle, we can lift heavier weights, and therefore strengthen more muscle.
If that is the case, then surely we could apply that very same logic to the hex bar vs straight bar. If a person can lift more weight using the hex bar, those same principles must apply. And for the purists, that say "well, of course you can lift more weight... there's more quad involvement... I don't want quad involvement. I do squats for quads. Fine... then surely the RDL or SDL is a better exercise if you want to minimize quad involvement.