There is another myth that says just because something is mathematically unambiguous it also intuitively obvious. And yet the real implications may be lost in the complexity of the situation or problem. Comments can point out anything useful to the code reader.
Also, if you spend enough time staring at a piece of code you can usually figure out what it does. But that does not at all mean you can figure out what the programmer was really trying to do. His/her internt may have been quite different. You can also not figure out all the possibly erroneous assumptions made about the problem space at the time of coding.
Comments form a useful redundancy and sanity check. Being cutely terse in comments is as bad as the clever but incomprehensible code one liner.
You should add comments anywhere you can predict a reader may have a high probability of misunderstanding.
- Tom