I don't think adding a girdle hurts, but I'm not sure if it actually helps. Maybe there is a benefit from tying together the stock main caps to reduce the "flagpole waiving" effect, if that is one of the things that contributes to the stock caps cracking.

Switching the center 3 caps to billet 4-bolt main pieces on a stock 440 block didn't stop the new caps from walking at 600 HP. I'll be interested to see how long it holds up with 650+ HP.

My backup block has been machined for the old style aluminum main caps and I bought a girdle for it, too. It'll be more of a PITA to build the bottom end with it, but in my twisted perspective it still makes sense.