What is the pro side of a transit bus without a rear window? See detail.

No pros that I know of. A missing rear window allows the engine’s exhaust gasses to enter the passenger compartment.

People in the back who have fear of being watched or attacked from behind can put their backs to a wall.

In the example you show, I imagine there is mechanical stuff of some sort where the window would be.

@Zaku is correct. There is “mechanical stuff” back there, usually the AC system on NYC busses.

There are shuttle buses for hotels that have a “back-door” for luggage storage. No window and a camera for rear visibility.

@kritiper As an aside , wouldn’t that be a defect if it allows exhaust gases to enter the bus?
@Zaku (I don’t understand your 1st statement.) and @dabbler but that’s not a good design, correct?

@flo I think it’s a good design to have the AC machinery at the back.
It’s close to the engine that makes the power to run the AC. In a large city bus that’s where the engine and power train are anyway. It takes up room in the least desirable part of the bus with the worst ride (way behind the rear wheels, any bump gets magnified) and with the most noise from the engine (back row).
I’m curious why do you think that’s a bad design?

@dabbler Why do vehecles in general have rear windows?

@flo Why do you think vehicles in general have rear windows?

I think vehicles that have rear windows have them for various reasons, depends on the vehicle.
In most ordinary passenger cars the rear window is as much for driver visibility as anything else, hence the rearview mirror in the middle of the windshield.
That usefulness is vanishingly small as the vehicle gets longer and as more stuff is put into the vehicle. A cargo van packed with stuff is too long for rear windows to be useful and the cargo blocks the view. Mirrors are how the driver can tell what’s behind. A bus is even longer and can be full of people and you couldn’t count on looking out the rear window for driving purposes.

I can see the “vanishingly small” part but I’m not sure it’s only the driver who can use the rear window.