Armor stat already effects the crucible. You concerns are misplaced here. Tripmines do not kill all guardians in 1 hit. It is dependent on your armor stat. Currently (excluding red death as I am not sure if they consider that medium ROF) medium ROF pulse rifles do 33 damage to the head. If two burst, all six shots hit the head that is 198 damage. The 2.5% reduction would be 32 damage to the head, all 6 hitting the head would 192 damage. With the reduction, this will only kill extremely low armor warlocks and hunters (I am pretty sure titans have a higher base armor stat.). In the currently meta, it is typically three burst to kill with those now anyway, (it's extremely unlikely all 6 will hit the head of a moving target) so I don't think the users will see much of a change here.