DC is not a great place to be on reserve for the Bus or 737 if you commute. The short call rules make allowances for cities with bad traffic such as DC and NYC, but it is conceivable to fly out of any of the three airports. BWI is sort of the odd man out when it comes to sitting reserve. You can get between DCA and IAD but there is really no convenient way of getting to BWI without a car.

Just to add on to what Andy said above, there are a small number of trips that originate and end out of DCA or BWI. When I was on the guppy, I would always try to get trips out of BWI then DCA then IAD as a last resort.