I am so glad that sisters of battle have a blanket 6+ invuln across the board because most of the times it is my only defense against the tons of ap3-1 weapons i face on the table.
The enemy things he got a lucky pen into a explode on our exorcist but wait we still have a save! Powerfist marine's fist halted by the blessing of the Emperor! That last sister of a squad trying to hold onto the game winning objective is shielded by out faith from the last meltagun the enemy has to shoot winning you the game!
Sisters of battle have a better chance of surviving those high strength low ap weapons than marines loyalist or traitor. Also fighting thousand sons, tempestuous, and ravenwing is not fun but at least you have a save!