The Spirit Effect
While it varies somewhat, the Spirit Effect is generally alive and well. When Spirit opens up a route, high fare airlines like Southwest will often dramatically lower their prices in order to compete. So, even if you never step foot on a Spirit plane, you will often benefit when Spirit opens service on a Southwest route. Would not be surprised to see some dramatically reduced fares on at least some SW flights from BWI-BOS, and probably to PVD and MAN as well.