To be honest, city attacks shouldn't randomly happen. As I said in another topic. Stormcloaks / Imperials are at war, yet you don't see them randomly attacking a city which the opposite sides control. You had a siege on a city, but even then all the other NPCs were safe from danger. Now, people tell me that this is for immersion Vamp/ DG when they randomly attack a city.
Firstly, I disagree. The first thing that should of been scrapped immediately was when someone mentioned the idea of a city attack. A city is meant to get quests, relax and equip yourself... not go Van Helsing style every time you wanna visit a city.
Secondly, if Vamp attacks are for immersion, why hasn't there been a single Imperial/Stormcloak random city raid ? Did nobody even think about adding a possibility of a planned attack like in the Civil War where you have to attack/defend the city depending on which DG faction you've chosen ? Apparently not

. It's quite simple. Plan an attack and have a welcoming party of Dawnguard members along with the guards to greet you, while no other NPCs are involved.
Thirdly, By adding this feature, I'm personally getting the idea that Skyrim is now focused more on vampires than on dragons. I strongly have to protest against that. DLC is supposed to be an add-on which builds up upon already made content, but does not in whatever way change the main path of the game. Killing off misc. quest givers is completely wrong and breaking the feel of the original. These kind of events are fun once, but what happens after a gazillion of times ? I found it even boring when I find that darn thief in Riverwood / Riften every time trying to hack'n'slash his way through, only to end up facing the dirt...