Back during the testing of Diablo 3, Blizzard said they were trying to empower the player with a wide selection of viable builds, rather than forcing them into optimal ones.
During a run of Normal mode, it is indeed true that every build is viable. However, during much of Normal, you do not have access to most of your skills or runes. It is really Nightmare where you can really start testing out new builds.
Even so, many runes aren’t unlocked until in the level 50s, well into Hell, where silly builds become unviable. By the time you unlock all possible skill/rune combinations, you are on the very doorstep of Inferno, where there are not a great many viable builds.
Without extremely high-end gear, optimal is the only viable way to play Inferno. Each class really has only a handful of these viable builds to have any hope of progressing through Act 2 and higher, without buying expensive gear.
Since this progression step can last a pretty long time for most people, it is important that there be more variety. Many skills have yet to see the light of day. How many Wizards do you know are using Ice Armor or Storm Armor (pictured) instead of Energy Armor? How many use Arcane Torrent? How many use the passive Power Hungry? If you don’t play a Wizard, you’ve probably never even heard of these skills.
And Wizards are probably the class with the largest number of viable builds at the moment. It gets much worse when you get to the other classes. How many Barbarians use Bash or Cleave instead of Frenzy? Ground Stomp? Ancient Spear? Monks using Inner Sanctuary or Tempest Rush? Demon Hunters with anything besides Hungering Arrow?
Heck, even entire setups just don’t pack the punch.
Yet a Barbarian or Monk with the same armor and resists will have a significantly easier time, due to the innate 30% damage reduction from all sources given to melee classes.
With Blur, the Wizard only takes slightly more melee damage than Monks or Barbarians, but takes significantly more elemental and ranged damage—42.9% more, in fact. This is because 100 to 70 is 1.429…
Sure, I might play my Wizard with a melee build just for fun, but at the cost of knowing I won’t be able to progress without switching to a more standard build. And I wouldn’t really want to farm on my Wizard either, when I can just farm on my Demon Hunter. But that’s another story…
The point is, without extremely expensive gear, there is no point in gimping yourself for Inferno by playing a melee Wizard, when you can just play a normal ranged Wizard or just play a Monk or Barbarian which are much better at melee with the same level of gear.
And this isn’t just for melee wizards—it’s for any sub-optimal spec.
Underused and weak skills need to be significantly buffed, adding some much-needed variety to Inferno. Blizzard is of course planning a balancing patch for sometime in the future, but let’s just hope they don’t get distracted by balance among the classes. A great deal of work should be done regarding balance of skills within each class.
Otherwise, due to the high difficulty of Inferno, which pushes even optimally built characters to their limits, there is little reason for deviating from anything that is mathematically optimal.