As such, I see no reason how practicality could decide the question of the use of the death penalty, as it seems to me just as practical (or even a smidgeon less practical, I admit) than real life imprisonment.
I myself have no defined position on the death penalty, but I know more or less the different arguments, and it seems to me they all can be reduced to two basic and incompatible opinions regarding what the punitive branch of the justice branch is for.
The first, and oldest, is the notion that the punitive branch goal is simply to protect society, what it accomplishes by removing from it those who violate (that) society rules. In this case society, which is what MUST be defended, is understood as a set of agreements on how one person should behave with another so that everyone can go on with their lives without causing trouble for each other and helping each other so that the end result is a net gain for everyone. So, if someone is being problematic, you punish him lightly so that he notices he's being a jerk and start behaving; if he does something more serious, you ostracize him temporarily (this ostracism can be literal in small enough societies, as you literally make the person get out of the village/tribe/whatever and taste living on his own, without the benefit of community support, so that he can start grasping how very much important being in it is -- and if he dies while "out", well, that's his problem); if he does something much more serious, you either ostracize him permanently (which, depending on context, is about equal to killing him), or disable him so that he cannot do that kind of damage to society anymore (this can take the form of amputation/castration, which, in more civilized societies, takes the form of lobotomy, chemical castration, inserting sensors to track the person every move, telling everyone who he is via public sex offender list etc.); and if his danger to society is understood be so extreme that you absolutely cannot take the risk of him remaining around, the next logical step is the death penalty, as it's the definite form of removal. Notice then that, from this perspective, being in any way nice to the criminal isn't part of the requirements at all. In fact, as long as he's being punished, or is an illegal alien, he's thought of as someone from without society, hence not deserving of any of societies niceties, which only actual society members deserve.
The second, and newest, is the notion that the punitive branch goal is to reform the criminal. This is similar to the above in many practical matters, since in many cases you remove the person from society, but the difference is that he's still considered part of it, hence deserving of protections, rights etc. As for the punishments, they're thought of as a means to an end whose focus is first the individual, and only secondarily society proper. As such, harsh punishments are usually frown upon and go unused, since they're not seen as conducive to any kind of individual reforming. As an obvious and necessary consequence of this point of view, the death penalty gets rejected, since if there's one thing that doesn't help one to become reformed it's being dead. Now, it's important to understand that this whole notion depends, to be valid, on the possibility of the majority of criminals reforming, which is something the defendants of the first approach don't think possible, or at least think possible only for a small minority of cases.
The current US system is clearly modeled on the first approach, with a few touches of the second one here and there. As such, even though on a moral basis alone it could reject the death penalty, keeping it around isn't incoherent, even the cost being the same. But where the overall approach to be change to the second one then sure, employing it wouldn't make sense, from any perspective, even a purely practical one.