Why? Ever seen one break? I have seen two cast modular cranks break in the nine years I've been tinkering with them. Both were due to excess rpm. Excess....as in over 8000 rpm. I broke one of them at 8200 rpm. Plus, it didn't break....it cracked and wobbled like hell. Great vibration at the top of third gear.
The cast crank does not have center counterweights like the forged Cobra crank. This is where the harmonics set up and begin to whip the crank out of shape. If you plan on rpm'ing the snot out of the engine (over 8000 every time you put your foot down) then I'd suggest the Cobra crank just for added piece of mind. However, for 99% of the street/strip cars you don't need to worry about the stock crank.