-Storm windows are there indeed if your windshield gets iced up and you can't see the runway. That's why most small aircraft with IFR capability have them. With cessna 152 e.g. you're not allowed to fly in icing conditions (no anti-ice installed) thus no storm window installed.
-direct vision panel serve the same purpose, except it's really located in the front of the pilot's vision. The BN Islander is one example i can think of.
-big airplanes usually have windshield and window heating, thus no such panel required.
-an airliner can be flown with windows opened. I've seen this done on a B777 and an A320 in hot climates. And there is a youtube video of such an event also.