If the client demands a cleared set because of confidentiality, then no escort. What is the big fat deal? Actually, the escort can wait outside? You have problem even with that?
Majority of the situation when an escort is involved has to do with private shoots, and that is what we are talking about. Escort are there because of concerns about safety. This is totally acceptable. Any other reasons, then it is probably inappropriate.
By the way, in a commercial shoot, the model usually feels safer because of the many people on the set, the make up artist, the stylist, the PAs, the air director, the client, the ad executives, and the list goes on and on. Still, with so many people there already, what is the problem with one more.
Personally, I don't like them if the escort is a bf/gf. I will tolerate them if they have absolutely no negative impact on the shoot. What is your beef with an escort if the model is concerned with her safety?