I would assume that those who can consistently afford higher end escorts / agencies would also be very cautious about their sexual health also.  Any sign of symptoms would increase the urgency for them to get it checked out thoroughly and claim to quit the hobby, but end up back into it after he's been cleared of possible STDs.

The only problem is that symptoms don't occur within the same day that an STD is contracted, so if you had multiple clients in the same day and one of those clients had seen multiple providers over a period of a few days, those symptoms may not have kicked in (if at all due to certain cases being asymptomatic), causing a possible transmission to a giver of unprotected oral.  This eventually leads into a chain reaction.

But there are also risks stepping out into the street and getting hit by a bus or delivery truck.  You can't just stop living your life because of risk.  Heck, it's a risk just discussing stuff on an open forum like this due to LE worries.  In the end, people are going to do what people do gage the risk to reward ratio in their brain and then act accordingly.  Taking precautions like seeing girls who test often is one mitigating factor though as you state, not foolproof.  If someone showed me scientific factual stats that told me that I have a high (ie more than 20% chance of getting an STD orally) it might change my mind, but not necessarily the minds of others to whom 20% is not worrisome, maybe 30 or 40 or 50-50 would be their tipping point.  What I am saying is, if you are in this "hobby" and don't realize there are risks, you are an idiot.  How acceptable those risks are is the question.