I don't either. Comments are only for the purpose of documentation, not explanation. If someone is reading the code and they can't tell from the code what it's supposed to do... then they don't really know the language do they? After all, that's what code is for... it IS an explanation of what the computer is supposed to do.

A lot of times, I will find myself wanting to put a comment like "has to be done this way because of FK in table X" - and I realize that what's really going on is a case of bad design, and luckily I'm usually in a position to fix that. If not, then yeah the comment is appropriate.