Well you still lose a strategic position by focus firing. It is a lot slower thanks to overkill (the game is still turn-based, even though each "turn" is approximately once every second), so even if you should do enough damage to kill an enemy unit, it wont register until the next "turn". This means that there is lots of wasted shot on overkill.
Also, I have yet to see, even in 3v3 games on Hard AIs, enough burst damage to kill a mid-high level capital ship with 1 hit. If it can happen, everyone should have more than just "big fleets" in their card deck to kill the enemy by then.
Since it is so difficult to 1 hit capital ships defended by a large force of support craft, that means that the only way to kill it is to concentrate-fire on the support craft, which can give the support craft's fleet time to kill the enemy capitals, since it has little or no support craft.
I do see the advantge to concentrated fire in small and medium engagements, but in very large fights it can drop the kill rate from 4 to 8 (or more) per second, to .75 to 3 per second and no capitals, which is a lot worse. (All estimated on a 1v1 maxed fleet engagement). Also in smaller engagements support craft can still make a smaller fleet destroy a larger one, it just depends on micromanagement and fleet-makeups.
And remember that support craft do not mearly heal/repair, they have other very useful abilities too, which support concentrated fire and damage.
But in any case, the side with more light frigates will have the advantage for capacitors in the late game. They are really a big deal!
However, all in all, if the "turn" game design mechanic was not in play, concentrated fire would be much much more powerful without its delay.