The 9/11 terrorists didn't photograph anything. Nor did the London transport bombers, the Madrid subway bombers, or the liquid bombers arrested in 2006. Timothy McVeigh didn't photograph the Oklahoma City Federal Building. The Unabomber didn't photograph anything; neither did shoe-bomber Richard Reid. Photographs aren't being found amongst the papers of Palestinian suicide bombers. The IRA wasn't known for its photography. Even those manufactured terrorist plots that the US government likes to talk about -- the Ft. Dix terrorists, the JFK airport bombers, the Miami 7, the Lackawanna 6 -- no photography.
What an interesting and valid point: terrorists aren't photographers.
Unfortunately, that doesn't mean that photographers aren't terrorists. Bruce Schneier's main point is that, "If we spend a zillion dollars defending Wimbledon and terrorists blow up a different sporting event, that's money wasted." There's an obvious flaw in this logic: the money isn't wasted, it worked. No attack on Wimbledon meant the protection put in place did the job it was there to do.
Sidenote: In the US, law enforcement and the military can stop you from taking photographs of certain things. i.e. bridges, nuclear power plants, the UN, etc.
Don't get me wrong. I'm a photographer. I've been stopped numerous times (by both public and private security) and asked to stop photographing. In every case, I was within my rights, and the officer had no right to stop me (though UK stop-and-search laws are rather annoying).
I'm no terrorist, and I do often struggle to see how pictures that I'm taking when I'm stopped could have helped a terrorist, nonetheless, if security experts think that photographs might stop an attack, then I'm willing to give them the benefit of the doubt and allow them to question people taking photographs. It seems to be a reasonable approach. As long as that remains their limit, then I am willing to continue being annoyed in the name of security.