I don't really use the focus target option at all. I see three possible uses for it...
1) Set the turkey to focus. That's the guy I'll be freezing. Then set up a few macros that allow me to send an arcane shot or distracting shot at the focus, while maintaining my target on the DPS mob that we're burning to the ground.
2) Set the Main Tank to focus. This will let me easily Misdirect to my focus. I currently have a Misdirection macro with a hard-coded name in it that I alter at the beginning of each party.
3) Set the Main Assist to focus. This will let me choose my DPS targets easier. My assist macro also has a hardcoded name that I fill in at the beginning of a party for this purpose.
I'm on the fence as to which would be best to go with.
I've never run with a party that uses a different Main Assist than the Main Tank, so for all intents and purposes, both my Misdirection macro and my Assist macro get the same name every time...the main tank's name.
What I really need is an addon that gives me little check boxes next to player unit frames, one for MT and the other for MA. At the start of a party, I'll just click to select the tank and assist, and it'll update my macros for me. That would really help, especially as I run into players that have goofy characters in their names and I can't type them into my macro. If I could do this, then I'll use focus for the turkey. I wonder how hard that would be to write. Perhaps something to consider once I'm Netherwing Exalted and I have a bit more free time.