Even though you have a broad range of spells, particular schools attack particular types of victim. As Jalkain has made clear, necromancy targets Fort saves; evocation targets Reflex saves and enchantment/illusion targets Will saves (though enchantment is probably the more effective in combat).
As such, even if you are a 'broad' caster, you end up using similar spells for similar victims. Casters generally throw necromantic effects at weak Fort saves and enchantments at weak Will saves, no matter how 'broad' they like their spell selection to be.
Incidentally, from a statistical point-of-view, Spell Focus is far more than a mere 10%. This is because of the nature of saving throws. If, for example, Darklich the Grim, necromancer, has a (non Focus) DC of 25 for Finger of Death and his targeted wizard/victim has a Fort save of +8, Spell Focus will halve the chances of his victim saving (he would need a 17 vs. the nonfocus spell and a 19 vs. the focus spell). And Greater Spell Focus would halve this again.
All in all, Spell Focus is well worth it.
