Abd al-Rahim al-Nashiri faces the death penalty if convicted of charges stemming the Cole suicide bombing, and from the attack on the MV Limburg French oil tanker.
Seventeen sailors died on the Cole in 2000 in Yemen; a Bulgarian sailor was killed in the 2002 Limburg attack.
Al-Nashiri's lawyers have provisionally had the Limburg charges withdrawn, though the United States is appealing that ruling.
In their motion to withdraw the death penalty in the Cole charges, the lawyers said "there is no military necessity served by executing the accused," adding that he is "far from the original theater of war in which he was captured" in 2002.
"There's no deterrence in executing Mr Nashiri," said military defense lawyer Major Allison Danels, adding that such an action would only inflame tensions and would likely play into the hands of Islamic State jihadists.
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"It would do nothing but enrage the terrorists and give them reasons for their brutalities," she said at a preliminary hearing at the US naval prison camp in Guantanamo Bay, Cuba.
Al-Nashiri was initially held for several years in secret CIA prisons.
"Even if Mr Nashiri is acquitted, he can be detained indefinitely by the US, he's no danger for public security," Danels added.
Military judge Colonel Vance Spath will decide on the motion later, but he acknowledged how long the case is taking to play out.
"This case needs to move forward, because it's (been) here for a long time," he said, noting that the US government was still appealing the dismissal of the Limburg charges.
Spath dismissed those charges this summer because he said the US military had not shown it had jurisdiction pertaining to the attack on a French vessel.
The judge declined to give a date for al-Nashiri's trial, though it was initially supposed to open in February 2015.