The problem with hidden blades is that they're either small and only good in a grapple, or large and easy to break. Assassin Creed style of hidden blades are flimsy; they're worse than butterfly knives with all their moving parts. And while you character may have hidden blades, Malakaus goes into battle in Full Armor. Sure most people think that being armored just makes it easier to dehumanize your enemy, but it turns out that stuff is good for not getting metal bits into your not-metal parts. Sure it isn't exactly subtly, but it's certainly effective. Contrast to popular belief, full plate armor wasn't even all that restrictive or heavy. If it was, it would be bad armor. I mean, armor that you can't move in? Who'd want that? Certainly tiring, but it's also why you don't have a lot of random people donning full armor into battle. It takes training, conditioning, and most of all time. You don't just have a natural talent for moving around with an extra twenty pounds of steel. That stuff needs to be conditioned onto you, like calloused palms.

A grappling fight is how most real life brawls evolve into. At the very least, you certainly don't want to give your enemy the space to swing his weapon which usually means you're right up onto them, either locking weapons or pulling him to the ground. Times like those is where a knife is truly deadly; that small blade might not look like much next to a big longsword, but it's just the rise size when your body-to-body with your current opponent. At least, so long as your enemy has some soft parts to put the blade in. That's where the armor comes in.