I recently swapped out an MSD distributor with Progression Ignition. I have been spoiled by installing distributors on cars without a back window. Doing the swap on a car with a back window was more tedious and took longer. And the quality of the wiring from the endeavor does not match that of the cars that I recently worked on without a back window.
Any wiring along the engine side of the firewall is difficult to maintain or alter with the window in place. Over time the wiring in that area tends to become a rats nest.
It is easier work on the fuel line and vapor line without the window.
Working on the throttle cable and mounting bracket is easier without the window.
For those that run Weber carburetors, a removable back window would be a benefit.
The back window is among the last parts of the car that I install when assembling a Pantera.