Documentation Is More Useful Than You Might Think
A common reason given for not documenting code is that the source code is accurate documentation—http://www.codinghorror.com/blog/2012/04/learn-to-read-the-source-luke.html; that, while documentation can be created with errors in it or can become inaccurate as the software changes, the source is guaranteed to be both an exactly accurate and exactly precise description of what the software does.
If you assume that framework and compiler bugs don't exist, then this idea is correct: the source is complete and exact documentation of the software's behavior. The problem is, it's not always the most appropriate documentation to read.
Sure, source code is entirely accurate, but it's also at the lowest possible level of abstraction. If you've just been brought onto a project and need to get to grips with the unfamiliar software, reading each operation in sequence (once you've even worked out the correct sequence...
