They'll hold shit together when your block splits in two.
Yep. Thankfully that never happened to me. I put far more than I should have to my stock block with a girdle. I don't know if it actually helped, but knowing what that block endured, I'd never build another without one.
Don't forget the cost of main studs and to have the block align honed if your gonna do it right. Personally, I think its a waste. Between the heads, intake and the oil pan all being bolted to the "now" split block......it'll all stay put. Just my $.02