Mike,
For what it is worth, I think protectors are a complete waste of time and money on the customer's part, but then if they are willing to invest so much for little, hey ho, on the carpet cleaners part.
We got 300 sq metres of new carpet fitted in our new house, mostly light, plain carpet and treated every inch with protector (god knows why) except for an off cut near a doorway which we treated only half of its width. We then deliberately stained it with wine, tea, coffee etc and left them for a month. Did the treated half clean up any better then the untreated half, no sir.
The acid test to me is this; if a carpet cleans up after a few years of use and the stains come out, as most do and the ones that don't wouldn't come out of a protected carpet, then why does the carpet need to be protected in the first place?
We do apply protector when after explaining the limitations and the above and the customers insists, but only under those circumstances.
Apologies if I misunderstood your point.
Simon