Here's an item from the Green Built Home checklist and the corresponding explanation from the verifier's manual:

C.35 (2) HVAC supplies and returns are fully ducted (no use of building cavities).
It is recommend that supplies and returns are fully ducted (using sheet metal). Using building cavities for duct work is not recommended because usage can develop large resistance to flow, has a greater potential for leakage, and air quality issues can arise -- as there's usually construction debris present and the wood supports could develop mold if wetted.

In addition to energy efficiency, we are also focusing hard on indoor air quality. In typical construction the air returns are bounded by studs and the back of drywall, i.e., the building cavities.  Matt worked with Modern Heating & Cooling to arrange for sheet metal stacks to be installed for these air returns, which we expect will give us cleaner and better indoor air.

Ring up another 2 points for a healthier house!