When calculating rebuilding costs for insurance purposes, should foundation replacement be considered?
Your insurance coverage should be sufficient to cover whatever loss you are expecting it to cover. If you carry homeowners insurance in case of fire, then it probably makes sense to ensure that the limits are high enough to replace the foundation and everything on it. If you carry homeowners insurance solely for liability reasons, it doesn't really matter what the property coverage is (you presumably have other ways to pay those rebuilding costs if needed). 

When I lived in Houston, I carried flood insurance on my house. It was a 2 story house in a 1000-year flood plain, so I felt comfortable with a lower coverage amount for the flood policy, as I was only intending the flood policy to cover repair / replacement of structures and property on the first floor - if my second story was flooding there would be far bigger issues to worry about than my house! This allowed me to save a bit on the premiums. My regular homeowners insurance, though, provided coverage for a full rebuild and personal property.