Our house has three crawl space doors on the outside of the house, and only one of them is in good working order. This one closes tightly and it has a little latch lock on the outside. This is good because the door stays secure and keeps animals from going into the space.
One of the three doors is actually a piece of wood held in place by a brick propped up against it. Actually, this works well enough--just not the most visually pleasing door I have ever seen. The third door looks like it was once a good door, but the hinges have come unfastened from the frame and the door is just hanging there.
In addition to an assortment of animals going under the house, the unsecured door also allows in a lot of cold air during the winter and this means frozen pipes if it gets really cold.