The problem is most of the arguments for tiered loot are based around the idea that it somehow benefits "the players". This kind of uniformity does not exist among players and in this specific case we are actually talking about a mechanic that benefits one subset of players only while the rest remains either indifferent or outright dissatisfied by it. Tiered loot is not about beating the highest challenge of the game as much as farming (best exclusively) a single (and hardest) difficulty.
* Tiered loot does neither increase enjoyment or longevity of the game for casual players who lack either in capability or inclination to effectively farm EE. They either ignore the higher tiers of loot (in which case tiered loot does nothing) or made peace with simply buying it (in which case tiered loot actually decreases the longevity of content since they have no loot incentive to run it on /any/ difficulty).
* Tiered loot does not necessarily increase enjoyment and does not increase longevity of content for the intermediate gamer (defined as all those who run content on a variety of difficulties). It actually adds a layer of (potential) frustration when after six elite runs a lesser version of the item they are looking for drops on the one hard run. They too are likely to simply go with what they get or buy what they don't pull. So, here tiered potentially adds frustration without having much of an impact on anything else.
* Tiered loot does not increase the longevity of content for end-gamers (defined as players mostly or exclusively concerned with owning the latest/best loot). It really doesn't take a (real) end-gamer that much longer to grind elite and once they have the loot there is as little point in tiered loot as any other.
* Tiered loot does increase the enjoyment and longevity of content for gamers who (exclusively or also) grind in-game items to sell for profit. Now, traditionally a different terms was used to describe players who grind gear solely for the purpose of selling them for profits and even taking into account it's not mutually exclusive it'd be easy to argue that when a player turns from acquiring gear to have the most effective end-game toon to grinding in-game items for real world profts they switch in category.
So, if I'd be to summarize:
* increases enjoyment and longevity of content for those who are willing to grind elite in-game items for profit.
* limits enjoyment and longevity for all casual players who want the best loot and buy it for AS and consequently have no loot incentive to run content on /any/ difficulty.
* removes some loot incentive from intermediate players (likely the largest part of the population) for similar reasons.
* promotes segregation of the playerbase. Players able and willing to run elites have no real incentive to ever join a group running a lower difficulty. Players running elites for profit have no incentive to burden with players from other groups as it cut into their profits by slowing them down and potentially be subjected to arguments about sharing or trading instead of outright selling. Players who dislike elite who even less incentive to try as they can anyway just buy it outright.
* fosters a greed vs need based mentality. Sure, your loot is your loot and you should be free to do with it as you please. However, that does not change that making items that can easily be sold for a nice amount creates and fosters a more selfish environment.
* whether or not you agree with the definition, the system still can easily be construed as Turbine creating a mechanic that both supports and encourages loot farmers.