Sure, valueless political discussions are valueless, and valuable ones aren't. Of course.
The question is whether we {do, should, can} condition our rejection of a discussion on its valuelessness rather than its political character. The implicit suggestion is that rather than treating political discussions as something special, we ought to enforce the general rule of rejecting valueless discussion.
The main difficulty I see is that we don't actually seem to enforce such a general rule.
I'd rather say that the "political character" of a discussion on an open internet forum like this -- one not devoted to politics -- tends to be a very good predictor that the discussion will be valueless, or worse. I'm sure there would be exceptions, but I don't know that this is shows much of an error in the existing karma system.
There is a tendency to downvote articles and commentaries with a political subtext with a remark on how politics is the mind-killer. I completely understand that nobody wants his mind to be killed, however, I disagree on the employed methods. I don't think anybody can really afford to ignore politics. It's a fact about any group of even a handful of people. Thus instead of shunning politics I think it's better to build one's rational defenses. Understanding that politics is a problem is only the first step. If you stop there, there will always be a big part of life where you are not rational. Therefore I suggest that, as long as it doesn't get out of hands, there should always be room for political discussions if not on the main site at least in the discussion section.