[Tench_Coxe]

"I can think of any number of execution methods that would have made a more significant point about suffering and dying -- impalement comes to mind."

Actually, crucifixion was a method of execution developed to extract the most suffering from the victim, given the technology of the time (as much as I loathe Wikipedia, it makes searching about easier, and the method described in this article isn't all that far from the historical method):
Cause of Death

[Tench_Coxe]

Ooops: here: Cause of Death