Both the therapist and the marriage counselor we see are busy enough that one cancelled appointment will not affect them that adversely. I have been my T's calendar (just which spots are blocked out by appointments), and his schedule is full. He has a list for cancellations and he fills the spot with someone else. I'm sure MC has something similar because we had to cancel one week because a court hearing conflicted with our appointment, and he said he would put us on the cancellation list in case something else opened up. The cancellation list he keeps is the reason he wants 24 hour notice if someone wants to cancel.