That's probably a side-benefit. If it had been an irregular pentagon (or any other shape), the inter-service rivalries would still be going on about who has the better office space (not kidding - you think vicious politics is only on capitol hill?)
Counterpoint by H.L .Mencken: "Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong."
They didn't stop building pentagonal and star forts in the Middle Ages. Some of the most important fortifications in US history are pentagon plus bastion star forts.