I deemed it important enough that when mine became so brittle that I couldn't use it any more, I made a new one out of a heavy duty trash bag & duct taped it into place.

A significant amount of water does get in around the windows and flows down inside the door & out through a drain in the bottom of the door. It's best to keep any trapped moisture away from the door panels until it evaporates or drains out.

In the old TLC I used to have, the drain would tend to clog. The water would accumulate in the door until it got high enough to seep through the door panel and saturate the carpet. It always surprised me how much water flowed through the door.