Biggest factor in my stiff policy right now, is me just being new... I don't have the client base to pay myself yet (ok, I'll be paying myself something for the first time this month). It hurts when I've got someone booked, to turn down a client that could only come in at that time, and then not have the first client show.
It happened to me two weeks ago, where a consult that booked two days prior didn't show, claiming she forgot about it, and I lost out on a two hour session with a client that could only show at that time. I got that latter client done this past Friday, but it's possible that, with the snow in the northeast, he might not have come in again until spring (he's an older guy that doesn't like getting out in the snow and he almost canceled Friday morning because of it, where the previous time he wanted to book, the weather was fine).