Asking for a client photo is bad business, but with today's social media climate, where "everything belongs out in the open," no wonder too many guys (especially younger ones) think this is OK behavior.
There are some guys who want to use the photo to determine personal attraction. Let's not forget, some horny service providers are in the biz simply to satisfy their hornyness, and figure why not make money as well. They don't really care if they turn lots of guys away, as long as one or two hotties send pics. In their mind, the request weeds out the undesirable.
There is a well-reviewed massage guy on this board who asks for a photo. I'm surprised no one has mentioned it. He claims he asks for it because he wants to know whom to expect at his apartment's call box. His intercom has a camera. He says he feels safer buzzing someone in whom he recognizes and expects.
Regarding those who fear the worst, anyone in the service industry who is determined to hurt you isn't going to last very long. All it takes is one leaked hurtful episode, and the guy will need to find another career, another source of income. Some guys are crazy, for sure (never book with a known drug addict), but if you shoot the wallet that feeds you and pays your rent, then guess what, you might as well shoot yourself dead.