Do I need Evaluate Intelligence?
Yes, if you plan on casting offensive spells you do. Without evaluate intelligence, your spells will most of the time do less than 10 damage. Mind blast and poison will be your only non-summon offensive spells without evaluate intelligence.
What Spells are affected by Evaluate Intelligence?
Almost all offensive spells (barring Mind blast and poison), as well as every spell that has anything to do with resisting spells (again, poison not included), meaning, every curse-type (as well as bless-type) spell, paralyze and mana draining spells.
The evaluate intelligence skill is primarily gained "passively" through spell casting. Gaining it actively through using the skill on targets takes a very long time due to the anti-macro code kicking in, requiring you to use, literally, several hundred new targets. So, you will likely be casting spells to gain this skill; it is vitally important to understand that any spell in the magery spell book has the exact same chance of gaining evaluate as any other spell. There is no difference at all. So, if you're solely looking for gains in evaluate intelligence, your best bet is most likely the 1st circle spell "magic arrow", as it only takes 1 sulphurous ash and 4 mana to cast. You may want to, if you're hunting, actively evaluate everything you combat, as it will provide a gain once in a while due to the new targets coming in constantly. That is really all there is to raising this skill.