So that is what focus fire does. It never was very clear by the tooltip.
Anyways, I guess that the preferred behavior for auto-targeting is already in place, its just that auto-targetting is so paced (that is, limited by the amount of time spent on it each frame), the non auto-premptability of auto-chosen targets, and ships can be so short lived in battle, that we rarely get to see great benefits of this behavior.
Its one of those things that the CPU is so stressed out, it can't fully carry out the auto-micromanagement that it could do given enough time. In these cases, you really have no choice but to step in and micro some yourself. Either the CPU has to step it up or you do, and the CPU is already near its limits.