Then you have incomplete experience with development environments. Debug/trace is so easy in VisualWorks Smalltalk, that a lot of people use it to comment by including example code snippets meant to be debugged. Yes, let me say it again: debugging is so easy you can use it to comment! For the right kind of situation, this can indeed be the best way to comment code. Sometimes no description is going to be as helpful as just seeing an example of it happen.
Does this apply to other environments? Not necessarily. In VisualWorks, it's usually no problem to spawn off multiple "hypothetical" debug sessions, with no consequence, even if you are already in the debugger. A lot of people also prefer to code in the debugger. If the cost of debugger use were different, then it would not be so.