If they focused only on FF then we would have one coming out every single year like Madden. FF would then lose all it's credibility and just be some franchise that's just updated all the time. Plus if Square never branched out we would never receive such amazing titles such as Xenogears (one of my favorite RPG's ever), it would just be so bland.
I do agree with you on FF XII and the fact that it took them nearly 5 years to get a new single player FF out on the market. But the biggest reason for that is not the fact that they were focused on other games, it's because FF XII had major development trouble's. I beleive the director passed out and had to be hospitalized because of over exaustion, eventually to be replace by a different person altogether.
Let Square do what they do best, and that's create amazing RPG's for all of us to enjoy.