It's in any case much more effective than physical locks for protecting information. The problem is that criminal investigators, for example, may very well consider my diary part of the investigation material. I don't think so. My thoughts belong only to me, and I'm not going to abandon them to anyone. The desire to read other people's diaries is, in my view, just a step on the way to the desire to read other people's thoughts.
Diaries are an improvement of one's memory, an extension of the intellect.
Yes, we're information-processing individuals, all right. And information technology is so many times better than a library ever was at storing and processing information. If you want to write anything hidden from the mafia, the government, or your family, you should use encryption. PGP, in one swoop, puts humanity's collected mathematical science between you and the superior powers. Encryption is a fact, and I suggest that anyone who wants a bit of personal freedom and privacy use it.