This is where we differ. When the shot is in a good spot, 3 and 4-blade designs still penetrate fine on medium/large size game. They work and probably a bit a faster with generally better blood trails (as long as they pass through). When you make a poor shot and hit a non-vital area - no broadhead will work reliably. In these situations you're depending on luck to hit a major artery. When you make a poor shot that hits bone, thick muscle or something that is protecting a vital area, penetration is the key to making a kill. The 2-blade design has an advantage here.

I have never heard of a person hitting a vital with a 2 blade design and complain that it didn't kill the animal. If they have a problem they usually complain about a poor blood trail. Myself, I'm very good at trailing, so I usually opt for the other advantages the 2-blade design give me.