So we pick a spender to build on. Let's say we pick Arcane Orb - Frozen Orb. This skill will be our primary damage dealer. We will kill the majority of the content with this skill. If we're running multiple spenders (say, Frozen Orb along with Energy Twister - Mistral Breeze), chances are we're going to run out of resources very fast and we'll be sitting around wondering what went wrong. This also applies to combining a traditional spender with a channeling spell. If we're running Frozen Orb with Ray of Frost, we have an issue. If we're channeling Ray of Frost, we're not using Frozen Orb. If we're casting Frozen Orb, we cannot channel Ray of Frost. So a skill is wasted. So generally speaking, we focus on one spender. Now we have to figure out how to sustain this spender to keep it going constantly throughout a fight.
This logic doesn't just apply for Wizards but for pretty much every other class there is.
The only time you have two spenders is if one is for AOE and one is for single target... even then it's not optimal because you normally use single target spells for bosses... which you can just change around your skill prior to fighting (like I use Impale against bosses as a DH).
I may have posted this before but this is how I build pretty much all of my characters:
One resource generator skill (some exceptions include stuff like Monk with the Combination strike passive)
One resource dumping skill (this is the skill that you dump almost all of your resources into for big damage. Stuff like Loaded for Bear Cluster Arrows, LTK, Wall of Light, Arcane Orb. These are usually the skills you get on your Boots and Helm.)
One utility damage skill (this is a skill that you cast once to activate for some utility purpose. You don't spam this. This is also usually a crowd control type ability. Examples include Cyclone strike, Monk Ally, DH's Sentries, Shield Glare. These skills you normally get on your Chest and Shoulders)
One defensive/mobility skill (this is a skill you use to stay alive or avoid bad situations or even move around. Stuff like Vault, Dashing Strike, Teleport, Steed charge even Leap and Spirit Walk fall into this).
One big cooldown ability (this is usually a transformation type skill or a big nuke type thing. Examples include Black Hole, Archon, Epiphany, Rain of Vengeance, Bombardment. You usually have this skill to complete your build, something that glues everything together)
One semi passive ability (This you basically turn on once in a while if that. It's usually just to buff you up. Stuff like Crusader Laws, Monk Mantras, DHs companions, Battle Cry)
I am pretty sure this is how the game was designed as. Sure there are exceptions but this is a good primer for new players.
