So what IS a rogue's purpose?

I don't think there is a design direction that rogues can take that allows them a place in a real raiding environment if they are not allowed to do good single target damage, as rogues by design (apparent in rogue mechanics) are limited to being effective only at dealing damage to a single target.

I believe think there is a lasting stigma that rogues SHOULD only be good at single target damage from the TBC days; but now due to Blizzard's new philosophy regarding pure damage classes they can't be too good, or other classes feel that they are overperforming. Due to this recent change in design philosophy, if rogue damage triumphs any other classes damage by more than a few percent, even in ideal conditions for the class, many people feel as if they are dealing too much damage and should have their output levels reduced.

So, as a long time player of the rogue class, I have to ask myself: If I'm limited to being a single target damage dealer, why am I not allowed to be slightly better at it than other classes who do not hold the same limitations that mine do? Why am I being punished and nerfed for dealing great damage in the one area that the developers have designed my class to do well in?

1. Rogues lack the tools to do damage to more than one target at a time.

Unless specced into Blade Flurry (Combat only), then rogue damage as both Mutilate and Combat is limited to a single target. Blade Flurry is on a 2 minute cooldown with a 15 second duration, so for situations where adds that last longer or shorter than this or multiple adds appear a rogue cannot do viable damage to more than one target. All other classes do not have this limitation, and have the ability to do damage to multiple targets at once (whether it be Whirlwind, tab dotting) while rogues, by design, do not.

Fan of Knives is not a damage increase to use on anything less than 3 (possibly 4) targets. If there are not 3 or more targets to hit FoK with, it is better dps to stay on a single target. Other classes gain damage from using their abilities such as Cleave and Heart Strike, as the cost of little to no single target dps. Rogues are the only class that has no such "free damage" ability outside of Blade Flurry, which is very situational due to its long cooldown.

2. Rogues are a melee based class, and are not viable for switching to targets a large distance away, and especially in short succession.

With no abilities that can do significant damage from a distance and the lack of a gap closer on a short cooldown, rogues are limited to their Sprint ability (3 minute cooldown) to switch to targets that may be out of range for them in many cases. For instance, on Professor Putricide, a rogue is the absolute worst class to bring to take care of the Slime Gas Clouds because they are limited to their run speed to catch them before the adds reach their target and cause massive damage to a player or the raid. While other classes can use their abilities to destroy the add from range or quickly close the gap on the target with abiltiies like Intercept, it is once again obvious that a rogue by design has a more difficult time doing damage to any adds that are a significant distance away. Especially if those adds spawn more than once every 3 minutes, a rogue is better suited to staying on target

3. Rogues do not deal very effective Area of Effect damage.

Due to the lack of long term, on demand cleave abilities and a rather weak AoE attack, rogues are even lamer in comparison to other class with superior aoe options such as Death Knights, Warrior, Mages, Warlocks, Shadow Priests. Each of those classes can either use their cleave type abilties in addition to their comparable amounts of single target damage for better damage, or may elect to produce much better damage through their area of effect spells. Rogues cannot compete with any class that is able to attack more than one target at once or posses better AoE damage spells in any multiple target situation

So I think it is pretty clear that rogues are still designed as single target damage dealers despite the developer's efforts to remove such a limitation from any and every class. Unfortunately, because of current rogue mechanics, rogues cannot and will not be good at anything other than single target damage. But one important thing to note is this: the other classes get to hold rogue-like single target damage but have the tools to do better damage in any type of situation that involves multiple targets or concentrated AoE damage.