The Effect of Deterrence

The Washington Post published an article today in regards to the puzzling debate over whether or not deterrence is a legitimate argument. As I was reading through, nothing came as a surprise or caught my attention...until the end. This article states both positions, which is ultimately stating that neither is justified as an argument. But the article concludes by saying,

A prominent line of reasoning, endorsed by several justices, holds that if capital punishment fails to deter crime, it serves no useful purpose and hence is cruel and unusual, violating the Eighth Amendment.

So, if there is no evidence that the death penalty is a deterrant to murder, then shouldn't the death penalty be eliminated? Here is the whole article: http://www.washingtonpost.com/wp-dyn/content/article/2008/06/29/AR2008062901476.html?referrer=emailarticle