i don't have time for a big response vet, but I'll say this:
i have never COMPETED equipped, but i trained for over a year with first a karin's double denim, then a grid-stitched double-ply rage-x, as well as a t-rex squat suit. i was able to squeeze about 130 pounds out of the shirt on a good day (in the denim i could get 465 with a 355 raw bench, in the rage-x only 455), i never had the equipment or the team to full-out try the suit, but i'd done 585 with it with no briefs, my normal work sets were 515 or so, straps down, for doubles and triples. at the time my best raw squat was maybe 435. on the deadlift i could get a whopping 140 pounds from it, sumo pulling 565 at my best when i'd never raw sumo pulled more than 425. i weighed 270 at the time, so my lifts kinda sucked.
i had this epiphany one day when i was visiting a friend in another state and we were talking lifts, i mentioned i'd gotten a 465 bench. the next day we were in the gym and he was hyping me up to another friend. but i didn't have my shirt and i had to explain that i can get 465 with my shirt, but only mid-300s without it. much as i tried to explain the deal of technique and how it's still super duper hard, the one guy said "oh so you can't REALLY do it", and my ego was pretty bruised.
i used all the standard analogies: shoes for runners, curved sticks for hockey players, better clubs for golfers, wet suits for swimmers, etc, to no avail.
a little while later i started losing weight, and my gear got looser. my raw lifts weren't down (actually my bench was better than ever), but equipped i couldn't do anything because my gear was a size loose. i'd have to spend another, oh, $450 on another set of shirt/suit/briefs just so i could still "lift" the same weight. if i wanted to move up or down a weight class i'd have to buy another wardrobe. if my shirt had a run in it (my denim shirt popped at one point), my training was hosed until it got fixed.
i started questioning things. if what i want to do is get stronger, if it's "power lifting", why am i constantly a slave to my gear? who's the lifter here, me or my shirt?
if you look at it as more of an "athletic spectacle" then yeah, i can see why multi-ply lifting doesn't irk you at all. after all, the bar is on YOUR shoulders, in YOUR hands, and without YOUR effort it isn't going anywhere. but to me lifting is just as much an exploration of the human limits as it is a sport, i want to get STRONGER. if my bench goes up 50 pounds after i bought a better shirt, did i get any stronger? no, i just have a new shirt. i can put 200 pounds on my bench without actually gaining a single new muscle cell, and that's crazy. then if i gain or lose 10 pounds suddenly i'm screwed.
it's like the old "guns don't kill people, people kill people" thing. yeah it's a person that pulls the trigger, but without that gun and bullet no one's gonna die. yeah the lifter lifts the bar, but without the shirt and cast-iron wrist wraps that bar won't budge.
i'm not enamoured with single-ply lifting either. to me that's just geared lifting that's too much of a pussy to admit it.