If I remember correctly, the reason Ghosts can revive Guardians is because Guardians have light inside of them, and the Ghost used that to revive them. Take what the Ghost says on a moon mission (I believe it is the second one) when they find the dead Guardian there, and couldn't revive him: "not even the light remains." This shows that the Guardians need light. Now, I realize that this may cause issues (although solving the problem of respawning being canon) with Guardians be having to worry about anything but pain. But how did that Guardian not have light remaining? Think of the light like a soul. The reason (or, at least I think this is it) there are Darkness Zones and why Hive are so dangerous is probably because they destroy your light, thus your Ghost cannot revive you. Just my thought, thanks for reading.