Assuming it's early-mid-game and I have relatively few hydralisks and mutalisks behind my main force of zerglings and banelings, does it make sense to focus fire on the medivacs or just try to take out more of the things that can shoot back? Marines are appealing targets, given their high DPS-to-HP ratio.
If the answer is "no, don't focus on the medivacs," then based on this more general thread it sounds like there's no point on focus-firing at all, let units pick their own targets.
If it matters, I'm playing against the AI on very hard. I'm too chicken to try placing yet.