If their goal is to harm Romney, blackmailing implies that the taxes are very very heinous, even without releasing them. It also extends the story through many

If their goal is to harm Romney, blackmailing implies that the taxes are very very heinous, even without releasing them. It also extends the story through many more news cycles. If they just released the taxes people would quickly find out that Romney does what most very rich guys do: minimize their taxes through legal shenanigans. The story would be out of mind come November. By letting people's imaginations run wild about what is in there, each individual will create his or her own bogeyman. It's definitely below the belt, but may not be a bad strategy.