IMC, we use "injury effects" in two ways, which (a little bit) encourage spreading some attacks.
First - Wound Levels and Penalties. I never state HP, and I don't allow the players to do so either. You can only say "Fine", "Bruised" (lost 25% of MaxHP), "Bloodied" (lost 50% of MaxHP), "Battered" (lost 75% of MaxHP), or "Crippled" (lost 90% of MaxHP). (0 is "dying", but you can't speak!) Bloodied inflicts -1 penalty to attacks and skills; each level below that is an additional -1 cumulative - Crippled is thus -3.
Second - which 90% of the time only impacts PCs - any critical hit and/or hit that does 25% of your MaxHP in a single hit causes Lingering Damage. In combat, each LD is a -1 penalty to magical healing or regeneration. [More rules than this, but this is enough for this discussion.]
So tossing a
fireball into a bunch of mooks to soften them up, followed by a barrage of arrows to knock them all Bloodied makes for good play - all the foes are now -1 to hit you. Yes, focus-firing the BBEG down to Battered is nice, giving him -2, but it makes sense to weaken the others too while you're doing so.
In play, I have seen the PCs leave off attacking weakened foes because at -2 or -3, they just aren't really a threat anymore; it's more valuable to attack the next enemy. Also, as DM, most foes think about fleeing at Bloodied, and nearly all run at Battered.
Combined with
@pemerton 's idea (and my answer to it) of "Focus Attack: -4 AC, +2 accuracy" as a combat move, the PCs will be
highly encouraged to spread out and occupy/eliminate as many foes as possible. (Makes the "Hordebreaker" ranger ability and that weak d4+0 TWF offhand strike actually really useful, too!)
I
don't think allowing "Focus Attack" to the ranged PCs is a great idea, though, and there are probably many other unintended consequences. I'd probably throw on at least "requires Concentration"; I want to say it doesn't work outside of combat, too, because Snipers are powerful enough as it is, but maybe that's a "feature", not a "bug"!