Considering that the Valk is one of only two fighters in FS1 that can carry the Banshee, which rips through shields and hulls alike with frightening ease
Decent on shields(good by FS1 standards), it's marginal against hulls(slightly less damage per shot than the Avenger), and carries a whopping 1.6 energy usage per shot(Avenger is 0.3, Prom-S is 1.0, Kayser is 1.2), and on a ship with 1/3 less weapon energy reserve than the Perseus(same power output), it's quite significant.
Equipping double Banshees makes you virtually ineffectual against anything that isn't a fighter, though a mix of Banshee/Avenger is pretty good.
I would say that if the Valk could carry Prom-S, it could simply fight by outranging any canon turret gun on bombers, though sometimes you don't have the luxury of "stay > 1km, but < 1.5 km away". It would be a pretty effective mobile Trebuchet delivery system(and could 'theoretically' fire the same number of double-fire salvoes that the Perseus(or Horus) can, then run back to the nearest support ship for more, faster than any other Terran fighter).
Of course, neither were likely available at the time the GTVA made the decision to create a new interceptor.
EDIT: Kayser energy usage I quoted was too high.