Coming hand in hand with these legendary items are the issues of imbalance that they cause. In PvE, terminology has started to crop up that puts legendary and non-legendary DPS into two separate categories of competitiveness. Concepts exist such as "enhancement shaman are currently one of the top non-legendary DPS specs..." -- a category that encompasses only 10 of the 22 DPS classes in the game. In PvP, concerns about burst damage have arisen, which was a big factor in the nerf to DTR that came in 4.3.
The issues with legendaries
Legendaries cause a balance problem, and that's a problem that's been exacerbated by two things in Cataclysm: a horrifically wide spread of specs that can use them, with 12 specs or five classes in total having access to legendary weapons right now, and incredible ease of access to legendaries (for the heroic raider).
When legendaries were first introduced into World of Warcraft with the beloved Sulfuras, Hand of Ragnaros, they truly were a thing of legend. Forty-man raids were anything but accessible to the casual player, and this legendary required both a ridiculous amount of luck and dedication to weekly runs to achieve. This model persisted through Sulfuras to Thunderfury, Blessed Blade of the Windseeker and Atiesh, Greatstaff of the Guardian.
The Cataclysm legendary model has two points. First, legendary quest chains are guaranteed to start for every player who can wield the legendary. Casters get the quest after defeating their first Molten Lord in Firelands. For rogues, it's guaranteed when they Pick Pocket a decoder ring off Hagara. Second, legendary progress is limited only by time, raid lockouts, and whether or not your guild chooses you as a recipient. While the number of drops of Seething Cinders (or their rogue equivalent) may vary boss to boss, if you show up and raid weekly and your guild wants to feed you legendary items, you will get your legendary after a few months of dedication.
Why legendaries exist
The value of a legendary is a weird thing to judge. First off, what they were more than anything was a status symbol. They were a symbol that you were one of the most badass players on the server, someone who was dedicated to raiding and whose guild thought was awesome enough to get what was likely to be their only legendary item ever. Second, they provide a sense of immersion and lore. With Sulfuras or Ateish, you got these incredible items wielded by powerful lore figures. With Dragonwrath and Fangs of the Father, you get amazing, lore-focused quest lines that make you feel like you're actually making an incredibly powerful item. They also provided a tangible benefit to the recipient, from Warglaive's haste proc or Thunderfury's incredible attack speed debuff.
The first two aspects do absolutely nothing to imbalance the game. The third is what ends up being the issue; these tangible benefits of having a legendary are where legendaries cause balance issues. From Val'anyr's ridiculously awesome shield proc to the 270 strength and burst damage on Shadowmourne, these items have procs that make them considerably more valuable than same ilevel items. With the game's being balanced (for all intents and purposes) around classes having access to items with the same weapon DPS per ilevel, legendaries provide a blatant and intended imbalance.
Should they still exist in WoW's current form?
With legendaries not truly being legendary anymore, and because their existence tends to mess with class balance a considerable degree, I ask this: Is there a justification for legendaries to exist in their current form any longer? I say this as an enhancement shaman, a spec that has never had the same sort of access to legendaries that other classes have and knowing full well that if legendaries become a thing of the past, I will never have one in my WoW career. That's fine with me, as I feel the benefits of a legendary-free game outweigh the negatives of playing for five-plus years and never getting one.
I'm not saying to entirely remove legendaries from the game. As a concept, I think the lore-filled quests for legendary items are a vital part of any epic fantasy storyline and something that Blizzard could actually benefit from having more of. I just think the actual nature of legendaries as an exclusive item that gives you a discernible benefit over other players is at its foundation too chaotic and imbalanced an idea to fit into the current design of Blizzard class balance.
In my opinion, the best solution would be legendary transmog items, unique models that are as difficult or more difficult to get than current legendaries, making them visually appealing and workable as a status symbol. Additionally, they'd be able to solve for lore junkies the five-year-old question of why legendaries don't level along with characters and expansions, making new weapon upgrades that drop in raids remain appealing while providing a way to always use "Broxrigar's Axe" (or other future legendary weapons).
Brace yourselves for what could be some of most exciting updates to the game recently with patch 4.3. Review the official patch notes, and then dig into what's ahead: new item storage options, cross-realm raiding, cosmetic armor skinning and your chance to battle the mighty Deathwing -- from astride his back!