It's a feature, but its a feature that isn't working as intended because Bethesda forgot how terrible their AI is on NPC's and that every NPC in town would try to fist fight the Master Vampires as opposed to running away or cowering (like they should). If the NPC's would just run or cower and let the guards battle the intruders, all would be good. But bad NPC AI makes the feature broken.
I dunno. I kinda felt like the devs liked the idea that NPCs could die - and not just generic guards - named NPCs. It's the same thing with the dragons. I believe that they're supposed to be
that much of a threat.
I won't argue that it was an intentional feature, but it was a poorly-conceived intentional feature. As someone else pointed out (not sure if it was in this thread or not), the vampire attacks occur whether you wish to play out the DG campaign or not. The only way to stop them is to play out the campaign. But you're not always going to want to do that with each and every new game of Skyrim you begin.
Compare and contrast with the vanilla main quest. If I don't want dragon attacks, I can simply opt to avoid the main quest. No "Dragon Rising" = no dragons. I can't do that with the vampires. It's either play out the DG campaign, uninstall the DLC, or do neither and be saddled with random vampire attacks on towns. This was a terrible idea. There was no long-term thinking involved. No consideration given to the fact that players would eventually lose interest in Dawnguard and wouldn't necessarily want to play it out with each new character.
So despite this being a "feature", it's a feature that does need to be patched. One initial attack is fine as an inciting incident. But no more should occur until or unless the player makes the conscious choice to enter Dayspring Canyon and begin the campaign.
See, but if they're NOT patching the NPCs AI with the dragons (it's been like over 9 months since the release), then why would they "patch" it now - they're had plenty of opportunities, including during the development of the DLC, to decide if they wanted to do something about it. Either they want it this way or it is unpatchable. So, the devs are just doing with the vampires what they did with the dragons. They had to have gotten feedback from gamers about the NPC AI regarding the dragon attacks, and not only did they not change it, they implemented the same AI with regard to the vampires. My explanation for this design is that if a vampire menace sought to blot out the sun, shouldn't a threat of that magnitude force a hero to respond immediately? Like, how do you call yourself a hero and yet ignore these attacks? I think the devs intended for the hero to
have to address it, on the game's terms, not the gamer's. This is all speculation on my part of course.
I can uncheck the DLC from the files that load at the beginning of the game (on PC), thus forestalling the onset of the vampire attacks. Is this not an option for console gamers?