Interesting.
I have used quite a number of all these bodies in my work, and I feel this about it...
If I am not concerned that the figure do the 500 meter dash anytime soon, ie. it is gonna sit/stand/whatever, fixed for the most part in a given day, on a shelf, clothed and covered, I am not that against any body if the height and scale is ok. Sideshow is a good height body. There are ways to tighten loose joints, so all I care about is scale.
I would use a HT body for a superhero in tights, or when the figures needs that extra muscle look, but still only in clothing.
The new HT rubber ones are better for exposed arms etc. so that is a no brainer...
As for Medi bodies, unless I want a shorter person, then I stay away from them. They are GREAT bodies, but are not in scale always with the bulk of my collection. That is all. No interest in debating which is made better. They are all good. Volks makes great ones. I just care about scale and what is exposed and how well it works for a given figure.
I have made six Terminator 1 figures using a cheap body, that is an off brand, but is huge like Arnold, and with the Frontline head, and HT T850 clothing, you get a great movie 1 Arnold that looks GREAT on the shelf and is in scale.
If you want total posability with every figure, your work is cut out for you. I want figures that look good and will last and be what I want on a shelf. And, have some body quality or posture of the person I am trying to recreate.
That is all I want.
I say, use what body is best for a given figure. All the rest is debate and preference. Sure there are some quality issues, as in how well a body is made, I get that, but not if the better built body doesn't work for the purpose I need in the given moment. Then so much quality doesn't mean squat to me. I need THE right body then and there. Opinion after that is arbitrary.
IMHO that is.
Addendum:
I used the old, original 1978 Mego 12 1/2" body for my short line of Christopher Reeve Superman figures. I only made a few, of which, Pjam is one of the owners of. I accept that it is a very old, and non-posable body compared to the awesome engineering of today's bodies. BUT, for some reason, it works with that figure in a way that no other does for me. It conveys nostagia, as it is the old 1978 Superman body, so, check! Nostalgia factor...then, it has a larger scale slightly than any other body I have yet used, making the Reeve seem even more majestic when compared to another figure next to it.
I know it is a deficient body now...but it has an aspect that no one else seems interested in capturing, short those larger DCD 13" bodies...that is, of the true SUPERHERO! So, I used it. Anyone that has one of my limited posable Superman figures has yet to complain. I think they got what I saw in it too. Not everyone will.
Maybe I will make a body that suits this purpose better myself...but this is an example of why I use a given body for a given purpose. Ultimately, it is a totally subjective choice I am sure.
Just had to add that.