While the simple explanation that Klingons can get honor to mean whatever they want (see Worf's "discommendation" arc on TNG, for instance) holds some weight in explaining this, I like to think of Klingons as hunters, as much as warriors. When you hunt, you don't announce your presence to your prey. You hide until the moment when you pounce and attack. The cloak fits if you look at it this way, at least.