I'm not sure you can look at the movie as a "bomb" just yet for a variety of reasons. First, as has been pointed out several times, ALL movies have paled in comparison with The Avengers, which took in $400MM domestically in two weeks and has set a number of box office records. Even The Hunger Games hasn't made that in the United States in nearly 2 months of release, and it had most of that time to earn income before The Avengers came out. Second, let's look at a few other films that are in release right now. Battleship, which supposedly cost $200MM to make, made $9MM in its first night of release compared with Dark Shadows' $9.6MM. The Dictator has taken in just under $12.8MM in its first 3 days of release (and I believe was promoted more heavily than Dark Shadows - to the point that I was sick of the commercials). Dark Shadows took in just under $30MM in its first 3 days. To be fair, that was 3 full weekend days compared to The Dictator's Wednesday release, but I'm willing to bet Dark Shadows will still be ahead after 5 days, when both will have had a full weekend for comparison. In only 8 days, Dark Shadows has already surpassed The Three Stooges, The Cabin in the Woods and The Raven. Plus, as MB points out, this movie has a fair amount of potential to make money in the DVD/Blu Ray market, so I am not in any way worried about the "bomb" label just yet. I think when people think of films that bombed in 2012, John Carter is going to far surpass any other.
And, to answer why some people are worried about the commercial success, I for one am hoping that it's enough of a success that it will allow whatever hurdles remain to get the restored version of Night of Dark Shadows released. I really want to see that movie as it was originally intended. I would not be as opposed as others to a sequel, either.