The reason not to use ND6-TD6:
#1. Ships like the Defiant would effectively not be able to hurt other ships, they're so unlikely to beat the turret roll.
Defiant can launch 2 waves, not as bad as one wave.
#2. The standard 4B vs T2 gets significantly worse - Down to 3.5 attacks on average from 5, a reduction to 70%. This would neccessitate a rebalance of all carriers.
What exactly do you mean? 4d6 would be... 14 attacks average, minus 7 average from turrets, so 7 attacks. (this is if you take away shooting down bombers, which i did suggest). While 4(d6-2) is on average 6 attacks.
#3. 8B vs T2 gets significantly better - Up to 17.5 from 11.7, an increase to 150%
I'd expect 8 bombers to utterly destroy any ship with t2 in the first place, how often do you see a wave of 8 bombers anyways? This is what fighters are for.
In my system 8d6 would be 28 average, minus 7 average from 2 turrets is 21. Granted, its much better than the average of 12 you'd get from 8(d6-2).
#4. Potential damage for a wave of 8 against T2 goes to 48 attacks from 32. This is far too dependant on luck - Defensively, you can do everything right, not brace because you don't expect a wave to penetrate your defences, fluff your turret roll and get annihilated.
This will require an extensive rebalancing - if it even can be balanced! The Emperor certainly doesn't need to get better, and the IN's other carriers certainly don't need to get worse. This is why number one on my ideal system is that the standard outcome for most waves isn't much changed.
46, not 48. I see your point. I do like my system in principal. It does get wonky with big wave sizes like you point out. Balancing it out would actually call for a different turret system, which I'm sure some people would be happy with, one based on turret density.
Off the top of my head, you would see it in ship profiles something like... 1 turret per wave, 1 turret per 2 waves, 1 turret per 3 waves. Something like that.
1 turret per 2 waves would still be the standard t2 vs a wave of 4, it would be bumped up to t4 vs a wave of 8.
Or of course a small table could be created showing how many turret dice to use vs a specific wave size, and have columns for the different turret density, kind of like how there's the gunnery table for weapon batteries. So you'd see a ship profile be like.. Turrets A, B, C, D, etc. Look up the A column, or B column, etc. VS the size of the bomber wave, and thats how many d6 you'd roll vs the bomber wave.