I don't know what the idea with shield mitigation is, but its current effect is not to limit the benefits of focus firing, but to insure that focus firing is the most viable solution in most cases.

Hypothesis: Shield mitigation mechanics reward focus firing in most scenarios.

I think we all know why focus firing, on its own, makes better tactical sense than ship-vs.-ship one on one firing: Reduce the DPS of the opponent faster to tilt the scales of the battle as soon as possible.

Examples:

10 LRM vs. 10 LRM:

If both sides spread fire evenly, one side will win, barely, depending on who got the first shots off, etc.. Each ship whittles down its opposite # till death, then helps out another if there's time, which in this case there won't be. If one side focus fires, they'll take down 1 LRM at a time, and weaken the opposing force significantly before any of their number are threatened. If they also retreat weakened ships they'll improve the numbers even more. EG, if LRMs still targeted, with enough health left to jump out, jump out, the rest of the mob can continue working on the survivers of the spread-focus fleet who will have to retarget significantly healthier ships in what is clearly a suicide by stupidity.

Enter Shield Mitigation.

So, shield mitigation means that as more damage is poured into a target, less of that damage 'gets through'. Significantly, all damage triggers some shield mitigation, and shield mitigation has a 'cap'. There is a certain point beyond which adding damage just helps kill off the maxed-out mitigating target.

If you spread-focus fire, you trigger mitigation on all the target ships to some degree. There is DPS loss for all attackers. If, instead, you focus-fire, there is DPS loss to a ceratin point, but any attacker adding DPS above the mitigation saturation point is doing non-mitigation-increasing damage. Sure, its only 43% or so of 'full damage', but you are still tipping the scales of the overall battle by removing their DPS (by removing ships, eh.).

It would take a very highly curved shield mitigation mechanic to offset the rewards of focus firing.

Focus Firing:

Only one ship's hull repair, shield regen dealt with.
Lower enemy fleet DPS in the only way possible: by removing ships.
Actually have and use target prioritization, rather than let some 'balancing' mechanic dictate targets.
Least amount of shield mitigation triggered: In all but tiny engagements of 3 or 4 ships, focus firing will trigger the least amount of overall damage reduction because the battle will be shorter and will speed up as it becomes unbalanced in favor of the superior focus-fire team.

Spread Firing:

Looks cool.
Gets you killed.
Triggers the self-repair of the maximum # of targets.
Puts a pail of DPS in the deepest well of shield mitigation: The whole enemy fleet.

......

If Shield Mitigation is linear, it certainly promotes focus firing. Even if its curved, it likely promotes focus firing more often than not.

If it is linear: Each DPS added to a target adds X percent of mitigation increase. I think it works this way now.

If it is curved: Initial DPS add very little to mitigation. The larger the DPS, the faster the increase in mitigation. Ideally, to convince people not to focus fire, mitigation should be Zero with the average DPS load: One average ship shooting at you does nothing. Two average ships should do only ~1.5x damage (25% mitigation). Three, 1.7x (40%). Four, 2.0x (50%). Five, 2.2x, etc.. So that the point of limited returns is reached quickly and decisively. Most importantly though, the initial load should have no mitigation at all. Otherwise you are clearly rewarding people to pour damage into already-maxed out targets.

A linear arrangement cannot inhibit focus firing, and because it starts non-zero and has a cap/saturation point, is often a clear incentive to focus fire whenever possible. It isn't a disincentive because no matter where you put your DPS, in a linear system you will get a mitigation effect, so it might as well be into a focus-fire target. Since this starts at 15%! it is even more clear that spreading fire is nonsensical.

......

What I don't like about mitigation (besides being an incentive to focus fire (who needs an incentive?)) is that it seems like managed economics, like a forced disincentive against being tactically astute. Why? Would you design a football game that likewise 'encouraged' only single tackles? It just rubs against the whole mechanic of combat. Its also counter-intuitive. You'd think a ship would be able to 'mitigate' against a light load better than against a heavy load / multiple source. Maybe single ships shouldn't even be able to damage each other? Rather than add a counter-intuitive managed dis-incentive bandaid to the 'problem' (why is it a problem again?) of focus firing, why not add positive incentives to multiple attack formats?

Maybe AoE effects that don't stack, so ships are encouraged to spread out (like crusader, but unstackable). Or more ship-vs-ship benefits, like an ability to mini-animosity a multi-turret ship: focus the flak's shots on you, rather than your fighters. Or shots that take a long time to execute, but do hella damage. Somewhat like the assailant's range ability. These things create positive incentives to diversify away from simple blob focus-firing. Shield Mitigation doesn't. And even if it were curved to be effective, this would be a bad thing. How would you micromanage 50 ships anyways? The interface has to be there, or the AI, and the incentive has to be there. Taking away advantages without incentives, mechanics, and a spread-fire interface to occur to players as alternate strategies makes mitigation seem like a cheap shot. I'm glad it doesn't really work as it looks on the tin.