I had one when the children were small. It was ideal for me- it works both ways, they are not obliged to give you a certain number of hours and you are not obliged to work a certain number either. It is very flexible.
In mine, in homecare, it was do understaffed that the idea of getting no hours was laughable. People were more likely to be pressured to do 60+ hours a week.
I can see it wouldn't work for everyone or in every type of business, but choice is always good.