Focus is very useful indeed, but I found that its better to use focus for all your macros

I allways focus my main before playing and use a /cast [target=focustarget] spell for everything.

This is very useful since all your macros would work even if you're not in party, in a raid or BG. It will allso make it very easy to change main on the fly with just focusing another toon.

However you would loose your ability to use focus as a CC target. But since you'll never have a normal target with the [target=focustarget] macro, you could just use your normal target for CC targeting.