I normally get asked to apply protector, I dont upsell it normally.
What I do tell them quiet strongly is that a protector is no magic formula that is going to keep their carpet nice and clean. It buys time, time to go and get a towel to pad the spill, but they do need to act.
A protector will also keep the carpets cleaner when they vacuum :

. As it has covered the fibre it resists sticky residue from sticking to the fibre, releasing more into the vacuum cleaner.