hawkeyefan
Legend
The question is whether he'd genuinely have enough information to do so. Depending on the game world and how things work, this can go either way.
Example in point: RQ 3 character with cuirboilli torso armor and a Protection spell up. You can probably tell the Protection spell is up and know enough to have an idea how your weapon will do at penetrating the armor--but the spell doesn't have a gauge visible that will tell you how many points it is, and there's a lot of difference as to whether its 1 point of 5 points.
As you say, these are observable things. I think it would be strange then, not to provide the player with a corresponding amount of information.
In a case like this, where they should have at least an idea of how well defended an opponent is, I think it's best to share the details.
The character doesn't know the mechanics.
As Thomas provided in his above example, the mechanics are representative of things the characters do know. If we think of this as a translation... we take the information available to the characters and then we translate it into information available to players... then I don't see the issue.