Vented Crawlspaces are hiding placing for many critters. Having duct work in a vented crawlspace is bad enough with the high moisture. When the crawlspace door won't stay on or one of the many vents are broken what else could go wrong?
Properly sealing the crawlspace helps reduce the changes of this happening. Crawlspace encapsulation also reduces the moisture, preventing wood rot, odd and musty odors and many more issues found in crawlspaces.