WHY DUCKBLADE SHOULD BE RUSHED SECOND ON MF INSTEAD OF BC

so i tried the build didnt quite get the dusk blade at first/// then i accidentally used my q on a minion when i had duskblade adn the second shot happend to hit the enemy support... quess what happend... q applies duck blade passive and the person was so chunked by the empowerd q.... that i literally 1 shot the support