That sounds flippant, but it really isn't. Imagine this: someone doesn't like you and wants to harm you. They plant a USB stick with random data on it on you,

That sounds flippant, but it really isn't. Imagine this: someone doesn't like you and wants to harm you. They plant a USB stick with random data on it on you, or in your home. They call the authorities and tell a story about how you're colluding with terrorists, and the information you've been exchanging with terrorists is somehow in your possession. The hard part would be to get someone to believe the story enough to get a search warrant, but if you're in a position of authority in the government, this isn't unreasonable. Now the warrant is executed, your home is searched, and the USB stick is found. The data is examined, and the judge orders you to provide the means to decrypt it. Because you can't, and because the data is indistinguishable from real, encrypted data, the judge orders you held until you provide the keys. Is this theoretical? No. It really could happen. The UK is a very, very scary place to be if you ever do anything that might piss off someone in power.