Because the unit AI currently does something nonsensical, we should keep focus firing in and useful.
Focus firing is the downfall of real time strategy, period. You cannot replicate real tactical defense when it works. A fortified position simply doesn't exist.
In real life, when people are shooting at each other, they shoot back at the people shooting at them, ignoring someone to focus your fire on one guy would get you massacred. Realism of course isn't a justification for something, but game play is. In a turn based game such as civ, 1/1 casualties are the norm when assaulting a defensive position, regardless of relative army sizes. If you attack 5 defenders with 10, you'll generally take even casualties. Attacking with just 5 would get you nowhere. They'd kick the crap out of you with their defense boosts, and you'd accomplish nothing. Attacking with double their numbers would be a minimalist attack. Something that will usually work, but not always. If, instead, you attack with a thousand troops, you're still going to lose those first five. It's the absolute loss of those first five units that creates balance and tactics. You're going to lose troops, more in one place wont help you after a very finite point. This allows people to use partial forces to deal equivalent damage to a larger attacking force, and also makes the use in a single steamroller attack significantly less so than a multi-pronged invasion of reasonable sized forces.
In sins, this will never exist outside of harassment. A 2-1 battle leaves one side dead, and one side nearly or completely unharmed. You can attempt to outplay someone by not engaging superior forces and just harassing them, but you will never be able to take a small force up against a large force, and do anything resembling real damage.
Then there are the decisions one makes in what kind of fleet to use, and what sorts of things one can actually do with a game. A command cruiser? Poof, 20 lrm frigates shoot it first, bye bye command cruiser. Ok, so you bring a dozen of them to support 50 ships, and they stick around long enough to be more than amusement for your opponent. What kind of a command cruiser is that? You need one command cruiser to use the targeting ability, just one. You need 20 to actually get to use it after the first few seconds. If there were no benefit from focus firing, that wouldn't be true. The repair cloud ability, completely worthless. If there were no focus fire, that wouldn't be true. The balance between longer range and shorter range units that is impossible to actually manage due to force firing resulting in the destruction of shorter ranged units before they even get in range, that too wouldn't be a problem.
Focus fire limits the game. It does not give you something to do in combat, it takes away options. It's been the crutch holding the genre back since inception, the fatal flaw that keeps rts from replacing tbs as a thinking game instead of a reacting game.