Obviously the concept of hybrids/semi hybrid classes getting multiple tier sets is pretty intuitive. They fulfill multiple roles, therefore they get multiple gear sets in order to better fulfill those roles.

I will not contest the needs of these classes to have multiple choices. However, I feel Blizzard has erred in not providing the "pure" classes with at least two choices, if not three. Not all talent trees are created equally...

Take Warlocks for example:

---Affliction---

-Crit Rating is less than ideal for an Affliction Warlock as they largely do DoT damage. Even when Shadowbolting +crit helps very little in the grand scope of Affliction DPS(A case could be made for Improved Shadowbolt, but that a Destruction talent, but even still bang for buck +shadow is best)
-Almost exclusively use Shadow spells, therefore dmg/heal items are far less than ideal. Much like a healer Affliction finds itself wanting a pure stat, +shadow in this case, rather than the more versatile, yet ultimately wasted on this spec's +dmg/heal.

---Demonology---

-Naturally based around being a more stat heavy than the other two. Having more stam/int certainly helps as in addition to their normal effects they increase pet mana/hp, give added damage through Demonic Knowledge, and are increased through passive Demonology talents.
-For Demonology Warlocks the Felguard is a fair portion of his overall damage, therefore equip bonuses or set bonuses buffing pet damage/survivability would be very desirable.
-Since Demonology leans to neither Shadow nor Fire it would keep the standard damage/heal to maintain its versatile nature.

---Destruction---

-Like Affliction, Destruction is more specific in it's damage type. It deals the most in Fire spells, therefore dmg/heal is less than ideal. Pure +Fire would be very benficial...though in the longrun Shadowbolt will scale better so there is a dilemma there.
-While not ideal, Destruction Warlocks are much more likely to be willing to give up stats in favor of more crit, hit, and +fire. Stats could be slightly sacrificed to eek out a little more nuke power.

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I could make cases for the other classes(except maybe Rogues...)

Hunters: Beast gets pet AP equip bonuses. Marks gets AP. Survival gets agility.
Mages: Fire gets +fire. Frost gets +frost. Arcane gets +dmg/heal, extra int and spell haste(to maintain versatility)

While our need of separate sets is not as severe as the hybrid classes I do feel that Blizzard and most players have lumped the DPS classes into a "one size fits all" category, when that simply is not the case. I would truly love to see DPS classes get some semblance of choice in our tier pieces.

I'd hate to see frost/fire/arcane sets. Gem slots allow enough customisation for mages.

It still doesn't make up completely irrelevant set bonuses, wasted +dmg/heal if you only use Fire spells, etc.

It would be no different a situation than that which a healer deals with. Should you find yourself respeccing often then you could have a secondary gear set for your offspec, or go with the more generic Arcane set.

The alternative is wearing non-tier and often inferior or poorly balanced gear when you respec, so yes.

Do we really need Blizzard to do all our min/maxing for us? If tier sets were all scientifically perfectly itemized for your exact talent spec then there'd be even more competition over that 1 token per week per 25 man raid, and they might as well get rid of all non-set loot that fills the same slots.

A large problem exists with the non-set loot.

Namely the lower drop rates, lower ilevel, and lack of options in many slots.

I beg to differ. When a set token drops I have to fight over every other rogue shaman and paladin in the raid; when an enhancement-designed piece of mail drops I pick it up.

From a player's point of view, it'd be ideal if every boss in the game dropped generic loot tokens which we could trade in for exactly what piece of gear we wanted. Blizzard could just implement an item editor and let us itemize our own gear, even.

But something tells me a game like that would get boring fast. Gear progression and its associated challenges (calculating, identifying, and obtaining ideal equipment) are part of the MMO game. Moreover "poorly itemized" gear is an effective way of Blizzard allowing progression without massive gains in ilevel... by "badly" budgeting entry-level gear such as Kara/T4 and optimizing BT loot to the extreme, we enjoy our upgrades that much more.

They've given the warlocks seperate PvP sets for Affliction and Destro. I don't understand why they haven't done the same for PvE.

And Mages.
And Hunters.
Rogues I'm still not sure on.

I can see the point of differing tier sets for Warlocks. In some minor way, the Arcane Mage's set wouldn't focus on +hit because we're given 10% of it on the first tier of our tree like Warlocks, but it's a fringe spec at the moment, whereas I'd say Destruction Warlocks are a few times more common than Arcane-nuking Mages.

And for the record, Fire and Frost Mages value each of the important stats (+dmg, +crit, +hit) practically equally. I can see no distinct differences, except perhaps a tendency for Frost to want slightly more +crit than +dmg, because Frostbolt scales pretty poorly with +dmg.