I think the argument that people could misuse SHA-2 is bogus, because if people are rolling their own crypto, they can get themselves into a lot of trouble, not

I think the argument that people could misuse SHA-2 is bogus, because if people are rolling their own crypto, they can get themselves into a lot of trouble, not just with SHA-2(key || data). Indeed, that argument may cause people to assume that it's ok for application programmers to do cryptographic protocol engineering, which would be disastrous. A much more general argument is that most people should be using cryptographic frameworks (e.g., which implement TLS), and Adam Langley's thoughts about whether or not SHA-2 should be skipped should be aimed at people who are creating those cryptographic frameworks. But if we are giving advice to random application programmers, they shouldn't be trying to pick cryptographic algorithms to begin with, and the question of whether you should be using SHA-2(key || data) is the sort of thing where the Zen master would be hitting the student with a cluestick for asking the wrong question to begin with.