hehe just wait till you get round to doing it U'll hate me ^^

basically the advantage over the simple focus method is this:

you gain focus as a tool to use in game (to target particular enemies, or to focus a friend (if you are taking him into a dungeon, so you can heal him quickly)
no confusion (after the initial set-up ofc >.<) as you just move your mouse to whichever toon is still alive.
It's faster.. no need to press a button to shift the assists
and it's rly fun to confuse the crap out of opponents when you cycle through mains in the blink of an eye.. (oh main stunned.. move mouse to next client WHAM!!!)

it was rly worth the effort for me.. the ability to switch so easaly was the defining point for me, makes gameplay so much more enjoyable removes a lot of frustration for me

if however, the above isn't so important for you personally then by all means the focus method works like a charm