A dead ghost can't bring anything back. Also appears the the ghosts serve some purpose in guardians ability to use light. For the purposes of radiance, it would appear that the ghost is used as a focal point or anchor for the warlock to self rez. After all you do seem to come back where the ghost was and not where the body landed.
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Nope cause Toland, Eris, etc can wield the light and they all lost his ghost
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Again, entranced by the ghost. Not needed. Eris lost her Ghost on Luna, yet she can still wield the light, she has to just to survive her affliction. My best guess is that Toland, Dredgen Yor, Osiris all too lost their ghosts Yet they can still wield the light.