As Randy pointed out I believe it was more a result of style as opposed to trying to prevent splitting of the horn. I also believe most salt, spice or other bag horns had flat or domed base plugs to fit into their possibles bag more easily leaving more room for more essential items,,,,,,,,like a rum horn.
Seriously though, most base plugs evolved from their basic function, which was just to 'plug' the large end to stop from losing powder, into the stylist forms you see that occurred during the 'Golden Age' of powder horns. Now there were no doubts as you noted that a horn or two may have been dropped and had their horn cracked but, I believe the majority developed cracks in the horn for other reasons. Now if you're wondering why horns developed cracks in the first place perhaps that's subject for another thread?
Gary