Our issue is that we only have the kids 5/14 days (5 nights every two weeks), and we generally try to make good use of our time with them, so in the last 2 1/2 years we've used in-home babysitters probably less than 5 times. As a result, we have no reliable babysitter on call.
Luckily there is a drop-in daycare that we can use if we need someone for a few hours. That's come in handy in some cases where we wanted to do an activity that was only appropriate for one kid, and needed someone to watch the other two. But they're not open Sundays, so that makes it a little bit more limiting.