I guess that a build-up of air pressure at speed (assuming at least the driver's window was down) could also tend to blow the rear window out, as well as the body flexing due to torque?
Early on, the driver's door glass (as a minimum) had to be down during the race. Later, all side glass could be removed. I guess wind pressure could have something to do with requiring the straps on the rear glass and maybe to help retention in an accident as well. The front clips would seem to have less to do with wind pressure and more to do with glass retention during an accident or spin-out.