I suppose "at discretion" has become the standard phrase or shorthand they all use. I'd rather they said service levels and degree of intimacy are dependent on the client's cleanliness and personal hygiene OR I will not touch smelly rancid looking cocks.
I was sat on a bus in a traffic jam the other week bored witless watching pedestrians going faster than me. I got so bored I started imagining each passing guy was about to have a punt and I was the escort (yeah I know, well weird innit!) and how "happy" I'd be be to receive them. Frankly, I rejected out of hand every single bloke...... all looked a bit greasey, slobby, warty, scabby, etc. I was of course far too picky and harsh but I still reckon if those guys are prepared to walk about in public looking a bit unkempt and rough at the edges they would probably rock up on an escort's doorstep in that state too. With guys like that about, I reckon it's inevitable ladies will invoke caveats like "at discretion".