It didn't stop Andrew deploying to the Falklands and doing his job there, so why would it stop William? Equally the Queen served in WW2, as did numerous other Royals. Charles served when he was next in line to the throne and I'm confident had there been a crisis and the tub he was in at the time sent he would ahve done the hnourable thing and gone (assuming some courtier or other hadn't put pressure on the powers that be to have him moved elsewhere pre-deployment). The spams don't understand the first thing about serving Royalty so why take the ravings of the DIA as anything other than wild speculation that is way off the mark?
Perhaps they are just jealous because there is a willingness to serve in the Royal Family while their Commander-in-Chief sat out Vietnam in the ANG, and hardly turned up even then.
Many European royals maintain a tradition of military service; I worked with one once. Even a token 5 years or so sends a positive image to the public and helps maintain a sense of tradition, important in this day and age when our supposed elected masters are beneath contempt.