I've had an unpressurized system before, due to my not putting the cap back on correctly. The car overheated and boiled half the antifreeze out of the car. Refilled the radiator, installed the cap correctly, and the car maintained the correct temperature for the rest of the time I had it.
You do realize that even though your system is unpressurized that the plastic is still aging and is still undergoing hear cycles, so when you tighten that cap to drive through the Colorado deserts that you're going to start leaking from a dozen different places when that aged, heat cycled plastic suddenly gets pressurized, right?