It's basically like crit in World of Warcraft. I'm gonna make parallels between WoW stats and League stats to make this easier to understand.
Agility improves a hunter's damage dealt with attacks _(Attack Damage)._
Stamina improves a hunter's ability to stay alive through damage (Health).
Critical strike improves a hunter's chance to hit for additional damage on attacks (Critical Strike).
Haste improves a hunter's ability to use abilities more often (Cooldown Reduction).
Multistrike improves a hunter's ability to attack faster (Attack Speed).
Mastery improves a specific aspect of a hunter that works better against some enemies than others (Armor Penetration).
Ability crits in World of Warcraft don't crit for double damage there, it's only 1.5x, so for specs that don't have some benefit from crits, Critical Strike is less useful than Haste, Multistrike, or Mastery. Fire mages have Hot Streak, which gives them a free instant Pyroblast (their most powerful spell) if they get two crits in a row. Marksmanship hunters have Enhanced Aimed Shot, which causes their Aimed Shot criticals to restore Focus (their resource, which works like Energy, so they can use more abilities). Feral druids have Leader of the Pack, which heals them when on crit (basically Warlord's Bloodlust).
Do Frost mages, Balance druids, or Survival hunters improve their DPS from crit? Yes, they do. It's not a wasted stat.
Do those above classes improve their DPS with crit as much as they do with Haste, Mastery or Multistrike (or as much as a fire mage would from crit)? No.
If a survival hunter had a choice between two pieces of gear:
Chestguard of the Merciless -- Agility, Stamina, Mastery, Multistrike
Chestguard of the Fireflash -- Agility, Stamina, Critical Strike, Haste
The survival hunter would choose the first piece (Merciless) because it has the stats he prefers. But if he had the choice between Fireflash and nothing, he would choose Fireflash. He would also choose Fireflash over a Merciless piece with significantly lower Agility and Stamina.
The same applies to a Talon or Zed. Yes, they do improve their DPS from critical strike. But they improve their DPS more from Attack Damage, Cooldown Reduction, and Armor Penetration. They would rather have an item with the same amount of AD as Infinity Edge, but with the critical strike and Infinity passive removed and replaced with Armor Penetration and Cooldown Reduction.