I imagine the point is to raise awareness of Assange's position with people who care about the artworks, in an attempt to inspire others to campaign for his fair treatment.
But like really, would anyone pardon a crime because they were being blackmailed with the destruction of art? Would a parole judge really take that into consideration? Seems more likely to me it would make them less likely to help Assange for fear that it looks like the threat worked, but most likely of all they would just continue based on the law and ignore this guy.
No of course it wouldn't enter into it at all except that it gets people talking and taking sides. Suddenly it's news again, and public opinion does matter.