Impenetrability was not intended to mean indestructible.
Is it a bit confusing and/or "useless?" A little. However, what it represents is that a weak model cannot just rip panels off and kill it, bare-handed (I think that baseball bats would, logically, be considered natural attacks). Therefore, one needs a firearm to shoot holes, a melee weapon capable of finding a weakly armored area in which to throw the proverbial monkey-wrench, etc.
Impenetrability also represents a hardening of the model that certain weapons can take advantage of - like RL's and AC's. RL's and AC's are designed to punch holes through said targets, and gains bonuses to do so.