I've seen far too many people who were having trouble with gearing.
To new and casual players trying to identify what stats they need on a piece of gear, gems, or enchants is a puzzle.
It's like you're the host of a dinner party and you have to go shopping for ingredients, yet your guests failed to tell you that one of them is allergic to wheat, another is on a low sodium diet, and another is a vegan. While you might've made your best chicken pot pie ever, it's a waste because none of those people would eat it.
What does that have to do with stats? Well, I'm sure we've all seen the warrior wearing holy paladin gear or the hunter with a strength axe and intellect armor. They wear it because there's no in game education or guidance telling them that those items were bad choices. Sure, they could go to elitistjerks, wowpopular, or any number of class specific information websites, but until someone says something about it (usually in a rude manner), they aren't even aware that they're making these poor choices.
There should be a prompt upon trying to equip (or even roll need on) gear that has useless stats for your class/spec. For instance, a Fury Warrior trying to equip a plate item with intellect and spirit would see: "Warning - The item you are about to equip/roll need on has one or more stats that has no effect on your class or talent specialization: Intellect, Spirit. If you would still like to equip/roll need on this item, please click the [random raid icon]: [list of clickable raid icons]"
However, this doesn't even help with the royal mess that are secondary stats. To someone who barely begins to understand that agility is the best choice for their hunter, they won't understand why the shaman yelled at them in party chat for rolling need on - and winning - that piece of mail armor with agility and expertise. After all, it did have agility! What did they do wrong?
What about when this person gets to 85 and constantly gets put downs from others because they can only manage 5000 dps while Mr. Theorycrafter can push out 15000?
I've seen (and helped) far too many people who were having trouble with gearing. Why does Blizzard leave it up to the players to educate other players about core mechanics their game?
Also, why do we even need all of the secondary stats? To people who will claim "variety" and "customization", the numbers will never allow for a creative person to be successful. For example, you could stuff as much mastery onto your BM hunter's gear as possible and have the hardest hitting pet in the world but your overall dps will be trash compared to the standard.