Original Editor Analysis
For the Forsaken to be part of the Horde is not a terrible stretch of the imagination. It is really just a matter of fully understanding both the motivations for both sides to ally, and the reality of their current alliance.
First of all, the Horde is clearly aware of the difference between the Forsaken and the Scourge. The Horde has battled the Scourge in the past, and understands that the Forsaken are not mindless killers, but rather tortured souls afflicted with a debilitating condition not at all unlike the orcs’ former demonic curse. Playing upon this knowledge, the Forsaken have used the premise of desperately seeking a cure for their affliction to mask their true purposes. So, although the apothecaries and Sylvanas could be plotting evil, the Horde is not at all aware of it. They do not fully trust the Forsaken, but many – such as The Earthen Ring, the advisory council within the Horde – believe the Forsaken to be seeking redemption. Do remember that the Forsaken, while led by Sylvanas, do not necessarily think like her. It is obvious that many, if not most, truly consider their Undeath a disease to be expunged. Examples are the Apothecaries at the Pools of Vision, inside one of the bluffs in Thunder Bluff, which are solely dedicated to cure the disease and train others in the arcane arts, and Leonid Barthalomew the Revered, a member of the Argent Dawn, who is held in high esteem for his suffering and his courageous acts against the Scourge.
Furthermore, the Forsaken and the Horde share an enemy in the Alliance, while not being enemies themselves. As such, the Horde has gained an ally against its enemies by accepting the Forsaken into its ranks. And in war, a new, powerful ally is nothing to discard lightly. Although they later went to war, it is not so unusual for peoples which may normally never consider alliances to become allies in times of desperation.
Additionally, the Forsaken do not live on Kalimdor with the rest of the Horde races. Instead, they live in Lordaeron. So, it isn’t like the Horde races are living together with the Forsaken. While they will get Forsaken visitors frequently, there are few if any Forsaken settlements on Kalimdor, and as such, the races likely do not have very frequent contact with each other outside the battlefield. This is a very important factor to consider when analyzing the political situation within the Horde; with the Forsaken being across the ocean, on a wholly separate continent, the rest of the Horde likely feels much more secure about allying with them, as they are at no risk of any military betrayal from the Forsaken. The Forsaken can't launch a surprise attack against them, and yet are right next door to the core of the Alliance's forces and territories. This provides a number of advantages.
To begin with, because the Forsaken are a lot closer to Alliance territory than the rest of the Horde nations, the Forsaken are a much easier and more convenient target for the Alliance than the rest of the Horde. This provides the orcs with a decoy, or means of keeping the Alliance distracted in a conflict.
Secondly, there is the potential for intelligence gathering and the ability of Lordaeron to act as a source of advance warning of a potential Alliance attack. The Forsaken would frequently engage the Scarlet Crusade and other elements of the Alliance military, which could in turn provide access to Alliance lines of communication, and thus give information about Alliance activity.
Third, the Forsaken help bolster the Horde's few sources of arcane magic, as orcs are limited to the shaman and warlock classes, and Tauren can only become shamans and druids. Because they can become mages, warlocks and priests, the Forsaken are like the trolls and the blood elves, in that they could be indispensable in providing defense against Alliance magical attacks.
Finally (and perhaps most importantly), the North Coast and the associated Zeppelin route would be of enormous strategic value and importance. The orcs could use the Undercity as a staging area and means of obtaining reinforcements during an offensive down into Arathi, and over time the Undercity could enable them to control all of the territory north of the Wetlands, keeping the Alliance very effectively boxed in south of Ironforge.
To reiterate, the Horde on the whole does not trust the Forsaken. They know they’re undead, and probably can tell they’re not the most benevolent individuals, but because of their situation, and because the Earthen Ring put in their blessings, Thrall, Vol'jin, and Cairne agreed to accept the Forsaken as a part of the Horde. While they are allied, it is an uneasy alliance to be sure. They had everything to gain by making the agreement with Sylvanas, and nothing to lose by accepting it.
Still, there is the question of whether or not the Forsaken are inherently evil. Warcraft lore tells us that Arthas created the Undercity as a labyrinth of evil, yet Sylvanas and her followers are not allied with him. It is shown that with the Forsaken, it is much as it is in all races: some are true to the cause, redemption, or whatever it is they may strive for, while others are treacherous, deceptive and disloyal to the Forsaken, the Horde, or anyone else besides themselves. --first created by Baggins March 20, 2007
Numerous members of the Forsaken appear to be insane. Barring a few Forsaken, such as those in the Argent Dawn (if they can be considered Forsaken), the vast majority of undead act like madmen considering their state pre-death. They see humanity, dwarves and high elves (not just their immediate enemies like the Scarlet Crusade) as a pox and work tirelessly to eradicate them, and they even hate their former loved ones (see the quests Until Death Do Us Part and A Husband's Revenge). Augustus the Touched is utterly mad, and Sylvanas butchered Blackthorn's and Garithos' humans without any regard, and yet she is touched when the player gives her Alleria's gift (see the quest chain The Lady's Necklace). Such contradictions and radical changes to their mental state would indicate that most Forsaken are insane, to at least some degree. --first created by Mannerheim March 9, 2009