I've had positive experiences working with agencies, though generally agree that their business model depends on getting people through the door and out another.
I think they key to making it work is to set very clear expectations and set up a rate that continues billing for the provided value.
I have too. I didn’t mean to make it sound like it never works out. There are times when it does. It’s just that when approached by agencies a lot of them turn out to be flakes shopping around. Occasionally you’ll get a serious person on the other end.