Mike, there's several possible situations here; first off, it sounds likely that the plastic was put down in an attempt to block odors from too much moisture. However it could also be that the plastic was put down because of a concern with foundation movement, although if that is the case the soil should not be DAMP, just not "dry as a bone." Is there good drainage around the entire outside of the home? Do you know if the soil in your area is sandy, or is it considered a clay? Also, did you notice what type of bugs? How long have you owned the home, and have you noted any cracks in the foundation?