I have to agree with James on this one.
In our game, we explain "hits" as actions that reduce hit points, but not necessarily causing physical injury by cutting and piercing the flesh. For instance, a creature with 80HP is hit by an arrow doing 1d10+4 points of damage. In my book, that doesn't mean the arrow stuck in him. The act of the arrow possibly striking the creature for a solid hit caused the creature to have to exert extra energy to keep from being pierced by a deadly weapon. His HP's drop showing a sign of that weakening. Until a creature or PC is bloodied, they haven't really been hit hard yet. When a creature or PC drops to "0" HP, that was the blow that finally got through to finish the deal.
Sure, there will be some small cuts and slashes from time to time. So when cover is present, even secondary attacks are affected by the -2 penalty to hit.
"Your arrow strikes the Bugbear in the shoulder, now roll for secondary poison damage at a -2 for the wall's cover.... (dice roll misses).... Although the arrow struck the Bugbear, it didn't break his hide. The poison was not effective."
OR
"Your arrow may have hit the Bugbear! He strained to get out of the way, now roll for secondary poison damage at -2 for the wall's cover... (dice roll hits).... your arrow must have cut him, not enough to kill, but enough for the poison to start it's deadly effect! OR (secondary dice roll misses)... your arrow did not hit him solidly, but since the Bugbear had to strain to keep from being pierced, he appears to be shaken and shows signs of tiring. The poison won't be affecting him with that shot, but he knows that you have the skill to kill him even with the protection of the wall!"