If it changes once a day, it is indeed idiotic as anyone with a scanner can pick up the password for the day.

If it is different for each flight, and you assume the flight plan distribution network is secure (which I can't imagine it is, but...), then it does ensure that only the person receiving the flight plan, or someone who the person receiving the flight plan told the password to, will land that aircraft at DCA. That might make a little more sense.