A main girdle is supposed to help, what it also does is keep everything intact in case of a failure which is the number 1 reason i opted to get one, if it helps save the block great!!
If your building a motor then ok, but to save stock stuff is a waste. If a stock block is gonna break, nothing is really gonna stop it
and a main girdle helps the stock block out too right?
From what i've read it mainly only helps to prevent main cap walk. Most broken blocks i've seen crack down the front from the either the cam bearing area to the lifter valley or crank bearing to cam bearing or something along those lines.