The benefit is that my guys never really have a target, except for maybe the main if Ive clicked on something. So in like arena and stuff this alows me to disguise my target.

-- Focus is not reserved, so you can save it for other things. I save it for special targets, that may require quick action like wind shearing a healer. Without having focused reserved, I can be dpsing one target and just click a button and have a action done to the focus.

--- Character switching, when one person goes down, switching is just as fast I can move my mouse to the next characters window.

-- Downsides, changing targets real fast is subject to latency. The characters may not have updated your new target when your casting. It can be a small pain to setup, as the macros are quite large but I really do think it's worth it.