I feel like by the time Zeri gets Muramana, her W is strong and spammable. Nashor's Tooth's intention at first was supposedly to speed up the duration in which W would pop faster, but this was proven wrong (sadly). That's why Pekinwoof (the person I'm inspired of for this guide) stopped using it. I'd say that I run out of mana quite a lot because W is very versatile for poke, wave clear, and catching enemies out.