"When implemented and used correctly" is key though. The thing with these things is that all it takes is one short lapse and you can be 100% compromised (not ju

"When implemented and used correctly" is key though. The thing with these things is that all it takes is one short lapse and you can be 100% compromised (not just that one message). People get sleepy, are in a hurry, misunderstand something, can be fooled, or are just plain idiots. And you're never quite sure what the capabilities of your enemy are. Person-to-person on the other hand is simple, easy to understand, and while not entirely foolproof (or idiot-proof), it's a lot harder to screw up and when you do, the damage capabilities are more likely to be more limited. I'm also going to guess that Israel has a rather hard time recruiting supergrasses among Hamas/Gaza population after the last 15 years (which is different from the IRA, which was thoroughly infiltrated).