Here are my thoughts. I was scared of just using /target CharName. It just seemed a little messy, since guess what right after that you have to use /assist. Why not just put in /assist CharName. Then I started a second team and realized, I didn't want names in my macros. Also, I didn't want targets in my macros either, since an alt would accidentally attack a mob that attacked it if its main target dies. So I switched to [target=focus-target] for DPS and [target=focus-target-target] for heals. No thinking, no guessing, no button mashing, no nothing, just attack and heal. Your alts can never attack a random mob. They can't have pets go run off and pick up adds. They just do only what the main tells them. Also, no mob on the main, no attack. Which to me is nice. What if I accidentally pick a friendly on my main and then /assist, oops, all my alts can now, not attack. What if I pick a totem (destroyable item) on my main, the alts kill it, but then they have no target to attack. In my mind I would spend about 10-20% of the fight actually having to worry about what my alts have targeted. With [target=focus-target], I don't even think about it. Just attack and go.

Everyone has their own playstyle. One way is probably not better than the other. It is just waht you are used to. Things that can affect your targeting, latency, time from when you pick a target on your main until you push an attack button, and if your macros have too many conditions (harm, dead, etc). In real terms, you probably only change targets 2-4 times in a fight, about 8 secs apart. So that isn't changing targets that much, just give it time for your main's choice to get registered by Blizzard and sent back to your alts (like 1 sec or so).