One of the biggest advances in building science over the past decade is the research that proves definitively that vented crawl spaces are bad.
If you've ever been in one, you know it's true. They're damp. They're dark. Critters have easy access to them. The batt insulation falls down. The air smells musty & moldy. And the crawl space communicates with the house, so that bad air (and the occasional critter) gets into the living space above.