Dave
As far as I am aware a solvent protector will adhere better to a nylon than a water - due to its chemical make up (synthetic to synthetic) and therefore more effective. Although I have to agree, who would really want a nylon carpet protected anway! but...you'll be surprised who wants a carpet protected regardless of its composition!
The only reason I can think of why the yanks only apply water protector is that they are more concerned (aware) with any potential problem with solvent protectors (look at Scotchgard when it was withdrawn a few years ago), that they just make do with water based? Or is it because its cheaper

...hhmm.
I havent heard of 5th generation being protected at source (apart fron acid dye blockers being introduced - not a protector I know). Correct me if I'm wrong though, but doesnt 5th generation suffer from acute yellowiness caused by high alkali's? I suppose it makes sense therefore to protect it with at least something?? Thats my excuse anyway!
Lastly, the only other reason I can think of for treating a nylon is that if you imagine a synthetic fibre getting 'cut' by normal use i.e. tiny fragments on soles of shoes etc, then dirt will infiltrate these cuts and look dirty. Even though they are easier to clean, by applying a protector (say after cleaning) to fill these gaps (albeit temporarily) makes it stay cleaner for longer in between cleans?
Thats my theory anyway! mind you I have had a few bevvies tonight already, so forgive me if Im talking complete twaddle!