Among the practical arguments put forth by the Times:

Capital punishment gives them the martyrdom they crave, making them symbols of sacrifice to would-be followers rather than powerless, humiliated prisoners passing the decades alone and increasingly forgotten in a cell.

....if Moussaoui is indeed an important cog in a broad conspiracy, then he certainly has information that could potentially be useful both in further Sept. 11 investigations and in our fight against Al Qaeda, whether now or in 10 years. We may or may not get this information from him if he lives, although life in prison is a very long time. But we will certainly not get it from him if he dies.

...killing him would do nothing to stop future attacks or alleviate the loss of the past. That's the hard thing about the death penalty: The heart screams for retribution, but it is never enough. It is vengeance devoid of benefit.

I will add one legal argument: No one should be executed for what they planned on doing, rather than what they did. If you follow the defense portion of the trial, you will see the evidence that (1) al Qaeda wouldn't allow Moussaoui to participate in 9/11 (2) neither the FBI, the FAA nor any other federal agency would have been able to prevent the 9/11 attacks or a single death even if Moussaoui hadn't lied to agants after his arrest.