I could see the existence of a parse mob increasing rather than decreasing misconceptions. A lot of variables go into melee combat, and fighting a punching bag with a static level and static stats is unlikely to shed much light onto many of those. Results are not likely to be applicable to much.
As for apples-to-apples comparisons of different weapons and such, even that is not very simple. Between randomized damage rolls, randomized mitigation, and randomized avoidance and accuracy, it's very hard to find "fair" averages in any reasonable amount of time. There was a period early on when I became a dev when I tried to parse a very small number of things, running 3 identical test chars against 3 identical targets for around an hour at a time -- and getting final results that were off by 10% or more (I don't have any records anymore, but that sounds about right). You'd need to put in a huge amount of parsing time to produce a body of data that might start to look reliable for one combination of weapons/gear, just to get some info that doesn't really translate into the game as it actually plays out. And I don't think anyone can seriously claim that very many players are gonna put that much care into it all; most will go for 10 minutes, see a low parse, and scream about secret stealth halfling nerfs (without realizing that they are wearing a gate neck, and that they lost R:Focus while they weren't looking).
I, and I think at least one other dev, had thought about making some kind of semi-automated parse punching bags before, maybe with some options to alter their stats, but the thought always kind of fizzles out. Most stat values for important things like AC and resists are hidden from players, so it'd be unlikely that anyone would be able to calibrate it to be just like their mob of choice. And even if they could, with how all over the place damage goes (as noted above), any minor variation in DPS due to changed stats is going to be basically impossible to suss out without literal days if not weeks of parsing -- and people are far more likely to declare the parse mob or the mechanic to be broken when the difference does not reveal itself immediately.
Should the parse mob just have no AC, resists, or avoidance, and guarantee 100% accuracy on top, to minimize randomness as much as possible to give the best possible apples-to-apples comparison with DPS numbers that you'll never see in the actual game? Or should it be true to the average undebuffed exp trash mob, giving values that aren't widely applicable (especially when it counts) and don't really elucidate any of the little details? Should it aux tank you if you attack from the front? Should it give casters infinite mana? Should it have a tank mitigation parsing mode where it attacks you but can't kill you?
It may just be more headache than convenience. I'm not against it in principle though, I am totally for more accurate information about game mechanics and stuff! I'm not the one to decide though.