But that's just it. The extent of the problem isn't a "fact". So far, I've yet to experience a vampire attack on a city, and I've been playing for a fair amount of time. Maybe it's something I'm doing that others aren't, who know. Now, I'm not saying other people haven't had the problem...
My point is precisely this: an issue for one or two people gets posted here, then next thing you know it's being taken as a fact that Vampire attacks on towns absolutely break the entire game, and Dawnguard should absolutely never be bought ever.
I take your word for good but also the word of those who say they have half empty towns. From what I gathered as an observer, it seems that a minority of people is "blessed" and have very rare instances in which the vampires go on killing spree in towns.
Like most Skyrim bugs, they don't impact all the people at the same time. Would you believe? I experienced the "blood on ice" quest bug only on my fourth playthorugh so I didn't understand all the fuss about it.
I'm not interested in vampire or vampire hunting so, for me, the rest of the content is more important... The sheer number of bugs and issues (also introduced where there wasn't any, like Breezehome) made me decide against buying it. I'm glad I could make my opnion before spending 20$. Being an online product, there's no refunding so I prefer to be extra careful with what I buy online.