I used to think that I should always use focus-fire when controlling a bunch of units - i.e. tell them all to target the same enemy, in order to bring it down quickly.
However, I saw some replays from top players and it seems that they don't always focus their units fire. I've been thinking about it and it seems that focus-fire does have some disadvantages:
- It requires a lot of attention (micro) which I could have invested elsewhere, especially for longer battles.
- The AI might not be able to respond smartly when the focused unit is moving backward but other enemy units stay put.
- It might lead to "overkill" - too much damage wasted on a unit which requires less damage to be destroyed.
But it still seems to be beneficial overall... so when should I use focus-fire and when should I just let my units duke it out? Let's assume there's no specific unit I just have to take down (e.g. a detector), in which case it's obvious I should focus it.