By tech savvy people, yes. To many people who are not well-versed in cryptography, substitution ciphers and the like are both the only obvious solution, and see

By tech savvy people, yes. To many people who are not well-versed in cryptography, substitution ciphers and the like are both the only obvious solution, and seemingly difficult to break. This will include a fair number of terrorists (see gwern's comment above), and so is a worthwhile avenue for a security agency to pursue.