The way I see it.... the question for the justice system is... do we want a system that is focused around rehabilitation? Or on punishment?
The death penalty is basically "giving up" on rehabilitation and is going for a permanent means of punishment. This makes sense for repeat offenders where we've tried rehabilitation and it simply does not stick... but not for first time offenders -- though you could make the case for an exception if the first offense is extremely heinous, and the person was clearly the guilty party (like a public mass shooting or something).
The way our prison system is set up now... we claim we focus on rehabilitation, but clearly that's not what is going on my any stretch of the imagination. We're gravitated much more on the "punishment" side of things. So...
going with that philosophy... capital punishment seems logical... though personally I think it's the wrong philosophy.
The even colder reality is that our (USA's) judicial system is based less on rehabilitation/punishment then it is based on
finance... which is the real problem. A large number of prisons in the country are
privately owned and receive governmental funding
based on the number of prisoners. This means there is financial incentive to keep as many people locked up for as long as possible.
This shows in statistical data, too. The US has the highest incarceration percentage worldwide. We've made quite a successful business out of it:
http://en.wikipedia.org/wiki/List_of_countries_by_incarceration_rate
Combine that with the state of these prisons and the concept of rehabilitation seems laughable. Or at least it would be laughable if it weren't so tragic and depressing.
So I think the question of the death penalty is really just a dodge for the bigger issue. We should focus less on whether or not we should implement that... and focus more on getting our shit together with our prison system.