Very few, if any of us have the models to fully take advantage of a double FOC.
Well, I do. (unless by "fully" you mean "fill all the slots" which just a crazy strawman arguement)
I'm not some crazy model-meister, either. I have two armies, Tau and GK. Around about 3000 pts of each I think.
I have some Tau sniper drones. Never use them. Why? Because even though they're dirt cheap, they take up a heavy slot. And a Tau player jsut sin't going to take anything but broadsides and hammerheads with only 3 slots. Same thing with the skyray. I really like it, but I'm not going to ditch a hammerhead for it.
Elites slots, too. Presumably, in the next codex, you'll be able to make suits troops in some way. But for now, Tau players Elites slots are crammed full. I find myself taking super expensive bodyguards for my Shas'O so that I can free up a slot just to take some stealth suits.
For my GKs, it's much less a problem. I suppose it would be nice to fit in a few random cheap inquisitors in a few places, but that's about it.
But again, see what I mean? It helps out the old codexes, mostly.