The macro's are similar, but very different on one point.

If you assist a toon, party leader or focus, your slaves are visibly targeting something.
If you just cast at a toon's target, the leader's target or the focus's target, the slaves don't need to target anything.

The first case is a lot easier to debug having a slave accidently pull something you didn't want.

The second case is a ton stronger in PvP, because opponents cannot see what you're attacking unless they target the brain toon. You can have all of the slaves target the priest, but the main targets the druid. They might put pain suppression on the Priest. The second is much more deceptive from pvp point of view.