Marginal doesn't matter. The function of justice in society is for people to feel protected, without the burden of constant attention to the safety and well-being of themselves and their family. That sense of justice is often wildly different than any objective measure of justice.

In the context of wrong convictions, I think the question is whether people fear being wrongly convicted more than they fear a cold-blooded killer escaping a life sentence. And secondarily, how strongly wrong convictions of others effects their sense of justice and equality more generally, and thus the legitimacy of their government.

I'm sure this sense has changed over the years, especially when we were more segregated and different classes of people didn't feel that the treatment of certain classes could effect their own class. But I think it's become increasingly the case that the death penalty is having a detrimental effect on our sense of justice and equality. Part of that has to do with modern mass media--dead or alive, our killers are both omni-present and strangely distant. Also, unlike in bygone times we basically kill on-the-spot many of the criminals we'd otherwise be afraid of. For example, domestic terrorists.

So on the whole I would guess that the death penalty provides increasingly less benefit and incurs increasing social costs. The moral question is more definitive for some, but if we're going to be utilitarian about it, it's much more complex than a simple accounting of the statistics.