You begin with 1 "free" specialization and 2 "free" focus slots for Evocation. So you use both because you have both. After that, specializations are limite to the skill pyramid. That makes them less cost-effective after a point.
To get a +2 specialization, you need to have another +1, so 3 of them total.
To get a +3 specialization, you need another +2 and another +1, so 6 of them total.
To get a +4 specialization, you need a +3, a +2, and a +1, 10 of them total.
To get a +5, 15 of them total.
And so on.
So for higher bonuses, if you specialize, foci end up cheaper. But they have the drawback that they can be left behind, stolen or even destroyed so it evens out.