The biggest threat to carpet life is abrasive particle soil. A good protector makes vacuuming more efficient and thus vacuuming removes more of this abrasive soil. Carpet lasts longer. I have seen this proven in testing laboratories and demonstrated it many times.
If you are not convinced of this, I will send you some samples and instructions on how to prove it to yourself.
Solution dyed nylon does have dye sites. It is still nylon. The dye sites did not disappear. Pigments are used for coloring. Some of the dyes sites may or may not be filled. The solution dyeing process results in more stable color and allows you to use stronger cleaning products without removing the or fading the color, but the dye sites are still there and can still be stained.
Oily stains on solution dyed nylon are no different than oily stains on any other nylon. A good protector helps.
If you use a protector with acid-dye resistors, (Maxim Advanced) you also get protection against food coloring in soft drinks, sports drinks and so forth.
You might not see beading, but that is not really important in commercial situation. It is not likely someone is going to come and blot up a spill before it gets tracked into the carpet. But the staining material is still prevented from being locked into the fiber.
It nice to be on the same side of a discussion as Bill.