Capital punishment exists to punish crime, not to aid scientific study. Subverting the purpose of execution can create a perverse incentive structure, wherein a need for more scientific study may drive judges to sentence more convicts to death row than they otherwise would.
By being used in experiments, they may die in a more painful way. In death row, we already break some human rights, including the right to life. Do we really need to break more, such as the right to not being tortured?