Well, it's often recommended that all 8 coils be replaced at the same time to avoid multiple labor charges. If the coil failed for a reason (oil leaking into the plug well, extreme heat, etc) chances are good that the others are not far behind.
When I had my Taurus SHO, I replaced individual coils when they failed. About every 4 months I was replacing another coil, never the same one twice.
-Rod