When to explain poor performance in a Statement of Purpose
My general question: When is it a valuable addition to a Statement of Purpose to explain an isolated case of poor performance in important coursework? 


Among scenarios I can imagine, never.  You don't want to place extra attention on potential mild weaknesses in your application.

As mentioned in the comments, in your particular case one B in a grad class while you were taking 2 other grad classes and 2 other math classes with all As doesn't really look bad.  The normal interpretation would be this student didn't have enough time to get an A in that one class with all the other work.

For the general question, one situation you might want addressed is if you have a semester that really looks bad (light load and actually bad grades, e.g., F, D, C), maybe due to major illness or personal tragedy.  However, the statement of purpose doesn't really seem the right place to address it.  A good letter writer will know when and how to explain this in your references, so you shouldn't need to explain in yourself in your application (though you may need to explain it to your letter writers if they're not aware).