A better example would be a LC2 (and not just because I'm into them).
The biggest weak point on a LC2 has always been the two bolt mains. Before girdles were available it was unheard of to run high 10s in even a light weight car (by light, I mean around 3200lbs).
Today it is common to see a TSM car (3400lbs minimum, stock block, full interior "street car") deep into the 9s. Without a girdle every single 9 second TSM car would join the DOTC (drove over the crank) club - probably on the very first pass.
I'll give you a dollar to find me a low 9 second TSM car going that fast without a girdle.
Hell, TSM will probably be in the 8s before the season is over. The record just dropped to a 9.10, and another car with a brand new combo just went 9.20 off the trailer. Between those two cars, one will be in the 8s very shortly. If you told anyone in the Buick game that five years ago you would have been laughed at.
So no, girdles aren't a marketing gimmick.