I have 5 buttons.

1- /assist main
2 - /assist toon2 (in case the main dies)
3 - /assist toon3 (in case main and toon2 dies)
4 - /assist toon4 (in case main, toon2 and toon3 dies)
5 - /assist toon5 (in case main, toon2, toon3 and toon4 dies)

My usual fights just require that hit button 1 to have a target that I want to attack. All other spell keys just casts the appropriate spells and doesn't need a macro made.
If my main dies, I hit button 2, and all the other same spell keys just cast the appropriate spells.. etc.

This allows me to use focus for sheeps. Having 3 mages means that I can sheep 3 mobs. If it breaks, or if the timer runs out, I just hit the "/target focus sheep" button for all 3 mages, and all 3 mobs (or less if I killed any) gets sheeped again without losing my primary target.

I used to use /focus and I personally don't know the advantage of using /focus over /assist [main].

One disadvantage that I see for the "/cast [target=focustarget] XXX" will always attack the focus' target. With having a button only to /assist [main], I can have my 4 alts finish up a mob, while my main is attacking and gaining agro from another mob. When the mob dies from the 4 alts, I hit the /assist [main] button, they all target my main's target and attack.

2nd disadvantage is that you have to macro every single one of your spells.

3rd, you can't use focus for your sheeping anymore.