My answer would have to be that spine is most critical when dealing with the flight of fixed broadheads.

Broadheads will go crazy on a underspined shaft, because the arrow flexes so much that they spend much of the time turned sideways to the direction of movement. This allows the broadhead to begin to steer the arrow. If the arrow is too stiff, and your setup doesn't allow for a "straight ahead" release, then your arrow is going to start off flying sideways, the broadhead is catching air again, and things go downhill from there.

On the other hand, I think it's fairly uncommon for archers to end up with a completely unacceptable FOC. If you're sticking with common weight shafts in the normal cut range of 27-30", using a typical broadhead of 100-125 grains, and fletching with normal size vanes/feathers, then you're probably not TOO far from "good." I might be completely wrong with that last assumption, but I'll never know because I don't bother to calculate FOC.