I get approached all the time. I've had experience with them, all bad. First guy was supposed to keep the wash clean and were only supposed to use one bay at a time, they pay to wash and minimal hand washing/NO detailing in the bay. Not only did they not keep the place clean, they'd leave a huge mess in their area and left my paying customers no place to park and clean their cars. I showed up on a busy Sunday and all the cans were overflowing, they were detailing a car in a bay and had two more bays block with cars just sitting there staged. The guy got his workers to rush and get the trash emptied, but they showed they weren't keeping things clean. It took me many hours over several days to get all the trash cleaned up.
Second guy had workers who would spend an hour or more detailing in bays after being told over and over not to. The last time I saw him do it I told him I'd call the police next time and issue a criminal trespass warrant. Suddenly he couldn't pay rent anymore and wanted to use the place for free. A friend of mine has the next nearest wash and he jumped through hoops to make the guy happy by running electric and water for him, but the guy did the same to him wanting to use his wash for free. I haven't seen him again, but my friend had to have him trespassed.
Last people were leased the dirt behind my wash by the owner, I agreed to let them have an area on my concrete in exchange for picking up, but they only keep their area somewhat clean. I had all kinds of trouble with them parking three or four cars on my lot, detailing in a bay when it was busy, soliciting my customers, I finally laid down the law and told them one more tiny issue and they're gone. They still aren't cleaning anything except their area, but I haven't had any trouble in a while and they pay to wash their cars so I make a few hundred off them a month.
IMO it's not worth it, but for an underperforming wash it could work out well, especially if they can help your customers when there's a problem.