I can imagine using a vacant bus for driver transfer might be a little frowned upon on some circles. At 5 miles per gallon, risk of getting stuck, low bridges and so on. There's also an argument that a bus is an asset prepped at great expense for fare-paying customer use, not just to have lying around for driver convenience.
It's pretty rare it ever happens though, usually a driver only takes a bus for a changeover if the bus they're relieving is required back at the garage or is defective.