AbdulAlhazred
Legend
There were a few jokes about that during the design process.
Really, though, the mechanic is meant to capture the flavor of an alert, experienced warrior. While the rogue inspects a locked door and the wizard tries to translate the runes scribed above it, the fighter keeps his eyes open for trouble, his sword at hand, shifting his feet to stay ready to move. An experienced warrior would look at the three and immediately note that the fighter is a highly trained combatant.
Even outside of combat, you can see it playing out like this. A thief creeps up behind the fighter standing at the bar. As he slides up to the fighter to pick his pocket, the fighter whirls around, grabs the thief by the hand, and slams him into the bar. It's almost a Conan-style thing, the really skilled, warrior driven by training that has transformed into instinct.
Still curious about the design space/compatibility issue. It seems to me that when you have classes using really significantly different mechanics such as existing martial classes and the Knight aren't you REALLY restricting the kinds of feats and items (and other add-ons) that will work OK with both of them? This would be an issue for both compatibility with existing 4e AND could be a real hassle for people designing these things going forward.