So, good thing or bad thing you ask? Well, this move will impact the design, operations and maintenance of those roads. It could result in raising speed limits, and precluding compete street enhancements like traffic calming features and pedestrian safety improvements. Federal funds can be used to maintain the roads once they're in the NHS however.
Transportation Issues Daily has more on adding to the NHS, and more pros and cons. Plus there's a link to a map to see what roads in our area have been added.