When you got such a bad machine print, you would not be confident that a properly exposed print could be any good, and thus hesitate to shell out more money to have a custom print made. Slides are good for mastering the exposure. From negatives, due to its color cast, it is not easy to judge by naked eyes whether a negative is properly exposed, unless the exposure is way off the scale. The wider tonal range adds another level of difficulty since, from the negative, you can still see most of the details. Slides are totally different.
Any subtle difference is readily observable.
From slides, now I see the subtle color rendition of different lenses. I never got prints from the same negative by difference orders that look even similar in terms of colors. In my opinion, if this type of prints is all that you've looked at, you are not qualified to talk about most of lens optics. Slides have different visual effects as compared with prints. For a slide, lights are filtered through it as if they were emitting from the scene.