The crank will break much easier with the main caps moving.
I'll use a friends car for an example, he had a girdled stock crank motor with billet mains. He beat on this motor for 10 years running 10 second ETs. It finally let go on the dyno, tossed the crank into three different chunks, nuked a piston so bad it was just chunks of metal. Even split the case of the transmission. The block was tested afterward and has since been rebuilt, the car currently runs high 9s with it.
Without a girdle on that motor there is no way the main caps would have stayed in for that much abuse. Sure, he probably should have had a better crank in it (and I'm pretty sure he went to an aftermarket one this time around), but that's a lot more money than getting a stock one that will live through some serious abuse turned. As far as I remember, back then there weren't all that many options for Buick crankshafts, either. Hell, there still aren't that many.
Though, with improvements brought around by people like my friend blowing shit up, stock block cars are almost in the 8s, whereas ten years ago getting into the 10s without joining the DOTC club was almost unheard of.
Anyway, enough rambling - the point is without the girdle you are MUCH more likely to toss the crankshaft. Grated, that isn't really the same problem as five liters have, since the weak link there is the block itself, but with Buick V6s, girdles are the shit.