Even the Joel article makes what's arguably a mistake: he says that input from users is "unsafe" and must be escaped on output, while strings from elsewhere shouldn't. That may avoid security exploits, but it still results in incorrect output when a predefined value really does need to be escaped.
The issue isn't whether a value originated from the user. It's the units/data type, as you said, such as plain text vs. HTML.
The issue isn't whether a value originated from the user. It's the units/data type, as you said, such as plain text vs. HTML.