They do nothing but add weight, cost and complexity. It’s no different that running a stud girdle on the junky GM stud mounted rocker system. I have dyno tested girdles and they will lose power because you are inducing flex to all the other studs.
Same thing with main girdles. If something is moving it will move whatever movement there is to another main. It’s a bad system.
If you can’t keep the main line in shape with what you have, buy a better block.
