The problem is that unless you have 10 or more in spell research fromt he city then 10% bonus just isn't very significant.
If you really focus a city hard towards one goal and then choose that bonus every time your city levels you can get a decent increase but unless you are pumping a fair amount of whatever without the bonus the bonus is too smal to make much of a difference.
I would love to see a "agriculture" city focus bonus as well.
