Well, honor by definition is always a bunch of hooey.
It's simple enough: you define what honor means, making sure it makes life easy for you and difficult for your enemies, and then argue that you meet the definition. If you don't meet it, you change the definition.
You don't necessarily aim to paint your enemies as dishonorable. Rather, you try to coerce them into behaving according to your honor code, lest they make themselves dishonorable. In the general case, your enemy isn't the soldier in the foreign uniform - it's the soldier in your own. You have to place arbitrary and downright niggardly limitations on how he can behave, in order to humiliate and control him, because a soldier free to act in his own interest is your worst enemy regardless of the color of his uniform - and one who has keys to your fortress is the greater threat.
Klingon honor is honor all right: from what we see, it's a code that makes soldiers unlikely to challenge their chain of command (except in the controlled manner of assassinating your immediate superior) but also likely to pursue victory in the most efficient manner available (victory is honorable, kicking of opponents when they are down is honorable because it's their own damn fault for being down, etc.).
It's also malleable, so that in lean times people gather around to worship the minutest whims of Kahlessian code, but in times of heroic victories they concentrate on those honor rules that pile up the Qapla'.