Lets agree that they are not very efective as PD, but I think that they have their uses at higher tech levels.
E.g.
Interceptor Mk2 class Interceptor 473 tons 22 Crew 195.4 BP TCS 9 TH 225 EM 0
23808 km/s Armour 1-5 Shields 0-0 HTK 6 Sensors 0/0/0/0 DCR 0 PPV 5
Maint Life 6.37 Years MSP 125 AFR 18% IFR 0.2% 1YR 5 5YR 79 Max Repair 112.5 MSP
Commander Control Rating 1
Intended Deployment Time: 3 days Morale Check Required
Boost 3 Magnetic Fusion Drive EP225.00 (1) Power 225 Fuel Use 1423.02% Signature 225 Explosion 30%
Fuel Capacity 10 000 Litres Range 0.3 billion km (3 hours at full power)
Gauss Cannon R400-17.00 (5x5) Range 24 000km TS: 23 808 km/s Accuracy Modifier 17.00% RM 40 000 km ROF 5
Beam Fire Control R24-TS20000 (1) Max Range: 24 000 km TS: 20 000 km/s 58 17 0 0 0 0 0 0 0 0
This design is classed as a Fighter for production, combat and planetary interaction
The hit chance is actually a bit better than it should be. Decent PD per ton, but bad PD per BP in my opinion. But it has it uses when protecting a flight of missile bombers (fighters) which are very unlikely to be targeted by AMMs and as additional protection for carrier tasks groups when on the defense.
Edit: At current tech levels a better BFC should actually help, it should hit 1-2 missiles moving at 40kkm, equal tech railguns should perform pretty similar (with a even larger R96-TS20000 BFC).
Interceptor Mk2 class Interceptor 499 tons 24 Crew 219.5 BP TCS 10 TH 225 EM 0
22583 km/s Armour 1-5 Shields 0-0 HTK 7 Sensors 0/0/0/0 DCR 0 PPV 5
Maint Life 5.90 Years MSP 127 AFR 20% IFR 0.3% 1YR 6 5YR 93 Max Repair 112.5 MSP
Commander Control Rating 1
Intended Deployment Time: 3 days Morale Check Required
Boost 3 Magnetic Fusion Drive EP225.00 (1) Power 225 Fuel Use 1423.02% Signature 225 Explosion 30%
Fuel Capacity 10 000 Litres Range 0.3 billion km (3 hours at full power)
Gauss Cannon R400-17.00 (5x5) Range 40 000km TS: 22 583 km/s Accuracy Modifier 17.00% RM 40 000 km ROF 5
Beam Fire Control R48-TS20000 (1) Max Range: 48 000 km TS: 20 000 km/s 79 58 38 17 0 0 0 0 0 0
This design is classed as a Fighter for production, combat and planetary interaction