In the context of brain function and recovery, the most relevant option to suggest why the damage might have been seen as a positive for Phineas is:
Gray brain matter can grow back.
Neuroplasticity, the brain's ability to reorganize itself and adapt by forming new neural connections, means that damaged gray matter can sometimes recover or compensate for lost functions. In contrast, white matter, which consists of myelinated axons that connect different brain areas, does not regenerate as easily as gray matter. Thus, if Phineas's damaged gray matter had the potential to recover, it could be viewed as a positive aspect of his condition.