I'm quite sure any qualified Dr will tell that that soapy water bath is completely useless once you contacted something at the back of her throat. And I don't think that most of the single guys seeing escorts do any testing at all.
Won't help with something in the back of your throat unless you deepthroat the soap.
Though it will help with a variety of things that may want to set up shop on your genitals, which is no small part of the battle. (Along with a good pee to flush things out.)
And I agree, too few people (single or otherwise) will get regular STI testing. That's why if you mention multiple sexual partners in front of a doctor they'll almost invariably (in my experience) test for common things, especially if you're otherwise having blood work done. I've been STI tested at testing clinics, and have been tested for some things as part of bloodwork at hospital, from my family doctor, and via walk ins.
And yes, some people don't see the doctor very often, and some STIs won't show on some people. However, most people who have something unusual going on with their genitals will seek medical help. It's my favourite toy in the world, if something was obviously wrong with it I'd run for help.
Like I said, if you're risking bringing something home that would surprise a SO, are torturing your genitals, are helping to cause antibiotic resistance, or are otherwise going to worry, strongly consider CBJ.