As described in this video, there are some big advantages to hybrid buses. Here you can see a bus that moves miles per gallon from the “normal” 3 MPG up to 10 MPG. It is a plug in hybrid that can go up to 100 miles before the diesel engine ever kicks in:

In addition, there is an unexpected side benefit to hybrid buses: they can be used as large, portable electric generators:

For Gustav-Level Outages, Cities Tap Hybrid Buses for Power

From the article:

Researchers at BAE Systems, a major vendor of municipal hybrid buses, are leveraging research done for the military to pitch cities on tapping power from mass transit buses. “If you’re going to have these power plants rolling around, it would be good to have them do something when they are not rolling around,” says Sean Bond, president of BAE’s Platform Solutions group. “The primary demand [for hybrid buses] was emissions reduction, and that has morphed into preventing greenhouse gasses and fuel savings. Another unintended consequence is the ability to provide power off the vehicle.”

The bus shown in the video has swappable generator sets for easy maintenance.