What you are saying is wrong. It's okay to say things you think is right, untill you are told that it's wrong. Then it's not ok to say it anymore. Ok, here we go:

All bosses have a lot of armor yes, but this armor ignore will help out a lot, because it ignores the boss' base armor!
Bosses' base armor is multiplied in encounters, so when you ignore base armor, you will also ignore the multipliers so it will "scale" with how much armor the boss actually has.

That's what's making these kind of buffs great, because you will suddenly be striking an enemy as if he was wearing cloth or something similar.