Actually, it was started by the manufacturers to reduce AC system
warranty repairs come the following summer (although a side benefit was
quicker defrosting). They figured you running the defrost through the
winter would keep the oils flowing through the system and keep the
O-rings and seals from drying out. Remember when you could buy a car
w/o AC? maybe not if your young enough, but I had plenty of them as a
teen...in fact i didn't have (working) AC until I bought a used 93
Saturn! hehe, anyway...just another, your van came with A/C so it was
designed to handle the added heat the A/C system would put on the
cooling system. Whether it's on or not, the trouble shooting steps are
the same.