I personally consider it DAMNED important, myself. My first intro to the dust cover's utility occurred in basic training at Ft. Dix, NJ in 1977. We were running from one range to another, preparatory to qualifying. It had been raining and we were going down a hill covered with red clay mud. I had my rifle (XM16E1, refurbished to M16A1 specs) at port arms, and some left handed guy ran by me and his front sight/barrel snagged my sling and spun me around like a top. I slid several yards down the hill on my chest with the rifle still underneath me. Upon getting up, I was a red clay mud monster with a diagonal green stripe where the rifle had been. Luckily I had a mag in it and the dust cover closed. I was able to clean the mud away from the sights, mag and ejection port area and qualified a few minutes later with no malfunctions. I would hate to have done something like that without a dust cover (closed) on my rifle. If all your shooting is done in clean environments, it's probably not important, but if not... I vote for the dust cover, myself. It should be ingrained as a habit to close the dustcover each time you handle the rifle, so as to make it automatic to keep it closed, IMHO.
Hope helpful,
John