Had a long discussion in the academy around this;
You can pack a pretty much unlimited amount of ECM on a missile to make it unhitable and deny the enemy any kind of AM defense capability.
While at lower levels this isn't a problem, at ECM 5 you just need 3.5 MSP to make your missiles impervious to an equal tech level enemy.
Seeing how that still leaves you with enough space on a size 10 missile to pack a reasonable warhead and enough fuel to reach targets at high ranges, I think this is unintended.
At maximum tech level, a Size 10 missile with 2 sizes ECM (thats -200% range/accuracy) can pack a good 90 points WH and still rather reliably hit targets moving at 100k on a distance of 200m, as shown in this example:
Missile Size: 10 MSP (0.5 HS) Warhead: 90 Armour: 0 Manoeuvre Rating: 83
Speed: 103800 km/s Endurance: 32 minutes Range: 200.0m km
ECM Level: 20
Cost Per Missile: 51.625
Chance to Hit: 1k km/s 8615.4% 3k km/s 2822% 5k km/s 1723.1% 10k km/s 861.5%
Materials Required: 10x Corbomite 22.5x Tritanium 35.176x Gallicite Fuel x2778.5
This could be fixed by multiplying ECM by 4 and divinding it through missile size, as I posted in the suggestions a while back, or by hardcapping it.
(or you could allow ECCM stacking, though I think that results in people getting several cheap instead of one good one...)
Regards