Short answer: very important. Like the attic space and living space of your home, crawlspaces require ventilation.
A crawlspace that is inside the building envelope should be heated, mechanically ventilated and should have insulated walls.
The construction industry did have some confusion years ago and many houses had heated/ventilated crawlspaces with natural draft vents also installed.
An easy way to tell if your crawl is over ventilated is if your crawlspace has insulated walls, a ducted heating system and natural draft vents you may want to consider blocking off the natural draft vents or you can consult with one of the experts at Temprite Climate Solutions.