Well, hey, if we cut 'em up into little pieces and distribute them to people who need those pieces, that solves both problems, doesn't it? You get your life reaping, I get my value extraction, and budda bing! Everyone walks away happy.
But on a more serious note, I think Genesis brings up an excellent point about the valuing of the victim. This is exactly why I once supported the death penalty, before I started considering the possible extraction of value from criminals. On a philosophical level it probably depends on whether the cost (in terms of lost work) of offing the criminal exceeds the perceived benefit of dealing out life-ending punishment, i.e., honoring the victim(s) in cases of murder, violent rape, and whatever other crimes we currently inflict the death penalty for.
Why just death row inmates? If we're going to take the step of assuming state or societal custody of cadavers, why not use every suitable corpse for organ donation, scientific research, or if nothing else fertilizer?
The way I'm reasoning this to agree with Thought's proposal, it's because criminals, as a matter of fact, are treated differently from non-criminal individuals. We have a right to liberty but we take that away from criminals in virtually all cases; even if we just slap a tracking device on them, we have removed some measure of freedom. If the criminal is placed on probation and given virtually full freedom, now that's something I can't apply that reasoning to.
Thought's proposal represents an extension of loss of freedom from "freedom of movement" to "bodily freedom." The punishment becomes more invasive, and doesn't really even have to involve death; we could, for example, force the removal of a kidney or lung from a death row inmate, which doesn't necessarily entail death. Consider it a compromise between life in prison and the death penalty. Although the question will hopefully be practically moot soon thanks to stem cell research, were this the 1950s, I think it would be an important thing to consider. Perhaps hundreds of non-criminal people's lives could have been saved if this had been implemented back then.