I read about (and have interviewed a few) "developers who can't develop". Is this phenomenon unique to the software industry, since it has no formal licensing boards and the technology changes so rapidly? I can't imagine doctors who don't know medicine or lawyers who don't know the law.

The lesson is that interviewers must give coding questions even to "senior" developers. See, for example, the "FizzBuzz" interview question:

http://www.codinghorror.com/blog/2007/02/why-cant-programmer...