Not sure I agree, for two reasons:
(1) Once you land at Dulles, it's a long journey to the main terminal (either by train, foot or "mobile lounge," depending on the concourse).
(2) Once you've made it to the main terminal and collected your bags, a shuttle bus ride is required to reach the rental car lots.
In contrast, National is a compact airport; the gates, terminal, and rental cars are close together. (Although that's not quite true for carriers in terminal A - Air Canada, AirTran, Frontier, JetBlue, and Spirit.)
I also note that depending on where in Fairfax one is going, National may be
much closer. Fairfax County covers more than 400 square miles - and the closest parts of Fairfax are only about 4 miles from National.