Point Defense Cannons are added by the Point-Defense AI tag ability. It does nothing other than allow the AI to add PDCs as a miscellaneous ability. You must add a new AI tag ability to the point-defense weapon that the AI does not currently use (just replace Point-Defense ability with it) and add this to the AI ship designs. Otherwise, the AI will only use the PD weapon that it considers to be the "Most advanced." Information on how the AI evaluates the "most advanced" component is located
*here*.