Li Shenron
Legend
A flat penalty instead of % chance looks more beautiful on paper, more "into" the standard rules, and avoids an extra die roll. Also what really matters at the end is the success/failure probability (or rate).
There is however a little beauty in having a separate roll, and it is with the fact that how it affects the shape of the probability curve. It affects IMHO more fairly the good and the bad combatant. When you have e.g. a character with 80% hit chance (from the normal attack roll) vs a character with 40%, if concealment gives a flat (e.g. -4 ie -20%), the weak attacker is more seriously affected than the strong one (60% is 3/4 than before, 20% is 1/2 than before). The separate roll (doesn't actually have to be a % itself) affects both characters chance equally.
You may actually argue that the better combatant should also be able to be less affected by concealment. But it certainly enlarges the divide between characters even more.
There is however a little beauty in having a separate roll, and it is with the fact that how it affects the shape of the probability curve. It affects IMHO more fairly the good and the bad combatant. When you have e.g. a character with 80% hit chance (from the normal attack roll) vs a character with 40%, if concealment gives a flat (e.g. -4 ie -20%), the weak attacker is more seriously affected than the strong one (60% is 3/4 than before, 20% is 1/2 than before). The separate roll (doesn't actually have to be a % itself) affects both characters chance equally.
You may actually argue that the better combatant should also be able to be less affected by concealment. But it certainly enlarges the divide between characters even more.