They can still cash in on fads. It doesn't matter if they exist in the lore or not, werewolves, vampires, and zombies are the current big fad in pop culture, and Bethesda is cashing in. Saying that these things are in the lore is just an excuse.
Note that I am not saying they don't have a right to do this, cashing in on the werewolf/vampire fad is just one of the reasons Dawnguard was released when it was; one of many. And frankly, it's not a reason I hold against them. If anything, it means Bethesda is doing research on their consumer base and targeting their content themes towards what they believe will make the most money. This money, in turn, is what pays for the next Elder Scrolls game. Or perhaps an actual QA department; one can dream.

I am as well getting tired of people comparing it to Twilight, I wish there was a working http://t1.gstatic.com/images?q=tbn:ANd9GcQhq934BSMdMKaM_U5VP-zm_mZAqeRaW82ov2HEKzV7tYtHagqxsg&t=1 to wipe the memory of that movie from everyone on the planet. As you said vampires,werewolves,zombies have been in the lore long before Twilight and they have been in RL lore much,much,much longer but in the life span of the Twilight saga it's been sullied and marked as a " fad " where as before werewolves,vampires,zombies where the stuff that nightmares where made of, the myth of each was enough to keep people indoors after dark AND the explanation for mysterious diseases, disappearings etc...etc... Beth needs to try and make it as gritty,bloody and dark as they can to counter this Twilight feeling people seem to have towards DG. 
) and a questline that is hopefully full of politics and social conflict, but could also be related to the Daedric prince Malacath, whom the Orcs worship. It would be a nice contrast IMO.