Obviously if you have other people in your practice they can cover for you, but like I said before: Many people do not like the idea of doing all their pre-natal care with one OB only to have an OB they’ve never met come do the delivery. There are also obviously many people who don’t care and figure if they like you, they’ll be fine with anyone you work with, and of course there are also many people who can’t afford to be that picky (e.g. if you’re on medicaid).