This is why I ask for a deposit for at least the first 2 bookings, after that then depends on how many cancels happen. I assess based on history, proportion and also if there are any special requirements such as significant travel, time since last booking, am I booking a hotel or tour.
Some on second bookings have left me sitting like a lemon, also learned to be strict with confirmation (people who I have extended a 'no need to confirm' have really messed me around).
I have some years ago had regulars who cancelled or changed over 50% of bookings made, although less common these days I also have the occasional log term regular start to cancel half of the bookings made, and or change the timing and or duration. Having to make a difficult decision now, because to ask this particular person to pay a deposit would (I think) really offend him but I have to find a way to manage it. Things do happen, but I know many do not have these things happen, and or do not change their booking around.
End of the day all things being equal I want to deal with people who are reliable, book without fuss and come to their booked appointment - which business or individual would not want customers like this!?