I'm thinking of when you take on The Guardian (bonus boss I know, but still, he's got the most armor in vanilla).
When you are fighting him, it doesn't matter how much damage your weapons can do, he has so much armor that you can only do 1hp damage per hit. But when you crit, you do the 2hp instead. So when going up against him, it's best to have all the critical hit-chance you can muster, and a really fast weapon (in my case a gun like the vortex beater). Wouldn't something similar happen with other bosses/enemies with a lot of armor?