Im sure accidents do happen where the window cleaner isn't insured, and the customer or member of public is a victim of that. Unfortunately what is likely to happen, as you havent answered the question I'll answer it for you, is the victim will have to suffer a loss of some description or another at the expense of the uninsured cleaner. Im sure you're just playing devils advocate, it's pretty obvious really, only a total idiot would seriously try to justify no insurance, and Im sure youre not that, like I say I doubt you seriously view this as a reasonable point of view but are probably just bored.