My first thought was aerodynamics, and indeed that is the reasoning behind this realatively simple design concept. There are several companies that market the skirts, such as the Duraplate Aeroskirt and the Trailer Blade by Strehl. There is an approximate savings of 7.5% on fuel costs, and with diesel now at around $3.50 a gallon, it would seem that the skirts would be a wise investment with the added benefit of reducing total emmisions. Several carriers have outfitted 100% of their trailers with this simple yet effective device. One possible drawback might be an increased possibility of a blow-over since strong cross winds cannot easily pass under the trailer.