The linked article isn't on sniping, it covers PTSD and the chemical changes the syndrome induces (I'm not smart enough on the science to do a chicken-or-egg disquisition). As a comparative analysis of combat experience logged since WWII, though, the article reasonably argues that our troops are the most experienced our country has had, ever.
Earlier patterns of combat were different. For example, during World War II, the bulk of the Allied troops in Europe went in after June 6, 1944. The fighting in Europe ended eleven months later. In the Pacific, the fighting tended to be episodic. A few months of combat, followed by many months of preparing for the next island invasion or battle. In Vietnam, not a lot of people went back for multiple tours, and those who did spend a year with a combat unit, spent less time in combat than they would in Iraq. Even during Vietnam, it was noted that many of those who were in combat for 200 or more days, did get a little punchy.
In Iraq and Afghanistan, army combat troops often get 200 days of combat in one 12 month tour, which is more than their grandfathers got during all of World War II.
And speaking of all things sniper, Stephen Hunter, the Fyodor Dostoyevsky of the sniper novel, has a new book on the streets. So, looks like everything stops on the Mongo reading list until I download and devour I, Sniper.