Why would a copilot's window be shaded/covered? I'm sitting in a small regional airport in Vietnam and I've noticed something odd about the aircraft I've seen arriving and departing. All of these aircraft appear to have the copilot-side window covered or shaded. (The windows are covered from the inside with what looks like the reflective covers you put against your car windshield if you park in the sun to keep the inside of your car from heating up so badly.)

All of these aircraft are ATR 72's, and they're all flown by the same airline. The one I just observed depart had the copilot's side window and cockpit window covered at the time they taxiied to the runway. The one I just saw land had just the side window covered when it taxiied off of the runway and over to the "gate." I can't speak to whether the window was still covered when the plane actually took off since I didn't have a clear view from the airport of the actual runway.

(What I'm calling "copilot-side" is the left-hand side if you're standing in front of and looking towards the tail.)

What is the purpose of keeping some of the cockpit windows covered? Is it a regular practice? (Is it safe? It seems like a safety hazard to cut your visibility in half while taxiing around an airport, even if this place is just an airstrip.)
Aircraft flight decks can become very warm at times and pilots will always try their best to avoid letting the sun add any additional heat. 

Most aircraft have sun visors or sun screens as part of the flight deck equipment. These are usually a clip-on design or pull-shade design. Both types are translucent and can be left in place for all phases of flight. 

Many pilots will also bring their own temporary sun shades to use in the flight deck. Theses can be translucent or opaque. Many pilots even use their charts as temporary make-shift shades while parked at a gate in the blaring sun. 

Since you described them as reflective covers, I suspect what you saw may have been some kind of temporary shade supplied by the company. They would not be used on the forward windscreen so they would be safe to use during taxiing. 

Here are some examples of a pull shade being used in an ATR 72.




Here is an example of what looks like a reflective shade.