It has been shown to produce juries that are notably friendlier to prosecutors than jurors in murder cases where the death penalty is not sought.
Studies have shown that juries in capital cases are more likely to believe that a defendant's failure to testify indicates guilt, more hostile to the insanity defense, more mistrustful of defense attorneys and less concerned about the possibility of convicting innocent people than a random sample of the population.

"There is a major bleaching of juries," said Samuel R. Gross, a law professor at the University of Michigan. "Many more African-Americans are excluded than whites. The biggest demographic predictor of attitudes toward the death penalty is race."

We like this quote from former Supreme Court Justice Potter Stewart in 1968:

Whatever else may be said of capital punishment," Justice Potter Stewart wrote... "it is at least clear that its imposition by a hanging jury cannot be squared with the Constitution."

In 1986, now Chief Justice William Rehnquist decided otherwise:

The Constitution, he said, forbids only the exclusion of distinctive groups like blacks and women. Barring groups of people defined only by shared attitudes is permissible. Justice Rehnquist rejected alternatives that might have addressed some concerns, like separate juries for the guilt and penalty phases of a trial or a single jury with extra jurors who could be weeded out based on their attitudes toward executions if a penalty phase was needed. The Supreme Court has since held that jurors who say they would automatically impose the death penalty for some crimes may also be excluded.

We'll end with this quote from former AUSA Jamie Orenstein, one of the principal prosecutors in the Oklahoma City bombing cases, now in private civil practice:

One of the basic ideas about the death penalty is that it's supposed to reflect the moral judgment of the community." Excluding opponents of the death penalty from capital juries makes gauging that moral judgment more difficult.