I def appreciate what you are saying.
My experience is this though... lets say, for me personally 135 is what i row for 8-10 reps normally at 90degrees.... If I throw say, 50 more lbs on the bar, my lower back is just fine, doesnt fatigue and i can keep my back straight... but the problem is... i just am simply not strong enough to row that weight! So my lower back is no where near the limiting factor there. The limiting factor is the strength of the target muscle.. which is kind of the idea
I understand what you are saying, but I think you are approaching it more from the perspective of just moving more weight... vs stimulating the muscle fibers as completely as possible. Less bend at the waist = move more weight, that isnt in question... the question is what is more effective. For me, 90 degrees is working well right now and i see the best development that i have ever seen in my back, but i have changed a few other things too (more deadlifts, stricter form overall, different movements) so its hard to say if 90degrees has done antyhing specifically for me or not, but i have to hypothesize that it has.