I like the theory behind the multiplier because you capitalise on getting the largest number of enemies on-screen to score points, rather than firing yourself willy-nilly -- I coded a similar multiplier in a previous shmup project (a combo meter for each enemy destroyed, the meter slowly depleting otherwise). But the current scoring mechanic -- with all enemy bullets on-screen turning into coins when an enemy is destroyed -- asks mostly the same thing of the player, to capitalise on the enemy layout and bullet patterns; so it might seem redundant to have both ingame.