You are forgetting the other 80% of scenarios friend. What if you loot a common/rare/epic/legendary/mythic and reroll into an arcane? Luck re-roll would help in this case. Luck reroll only doesnt do anything if you already looted an arcane item.
It's best to think of reroll as a second chance to get something better or of the same rarity if you have not done so already.
Ultimately it can only help not hurt.
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