Because in the time it takes to do 1 Turn run of Pharos Sirius, you could do 3, Turn 4 runs and get x3 the amount of tomes. That WILL make all dungeons below Coil completely redundant.
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They will probably reduce the amount of tomes you get, and there will most likely be a loot lockout to one item per turn.
But you can keep spamming it until you get the item you want.