I think this is the thing - all the missile solutions heavily overlap with both the 57mm and the two 30mm cannon that are part of the anti surface module anyway. I *believe* the turret carries 120 ready rounds but I could be talking tosh. Given the way the rounds shred targets, I wouldn't rate most FAC's chances of getting very close.
The FACs don't need to get close, seeing as how they are usually armed with anti ship missiles or more effective guns.
Against a FIAC threat, there are limitations to the gun solutions due to the engagement geometry problem. The missile solutions would overlap with guns, but it is complementary, not redundant.