"Permanent" has a very different meaning from "infinite amount of time."

Permanent implies that it's irreversible, and this is definitely not the case.

Given that the gain element must be employed for the storage of the light, and the removal of the gain element causes the release of the light, it is not "permanent" - it is "potentially infinite, as long as the gain element is in place."

That makes it useful for research into all-optical computation.

Theoretically, the gain element is not necessary. They only need it to compensate for physical imperfections in the device.