[bombcar]

Even midwife systems often end up with a "ball of midwives" where you may not even know who you're going to get, and then they go off the clock at 6 PM and someone new steps in.

Everything is a cost/benefit/availability tradeoff, and time is just one of the many variables that are being juggled.

Of course, if you have money to throw at the problem you can reduce the other issues.

[Xeoncross]

As far as I'm aware, the majority of US midwives (or CPM's) are not hospital nurse midwives. They do not go off-the-clock, though they sometimes have multiple patients due at the same time so you might still find a backup midwife is covering for your primary care provider.

[bombcar]

I'm sure it varies from birth center to birth center - one would have the midwife scramble at any hour of the day or night to get into the room, the other we used had the ball.

I can see the advantages to the ball but the first was overall better I feel (but there are so many variables it's hard to really tell). We've technically never had the doctor/midwife "assigned" on-hand during the actual delivery for various reasons.