It is simple: 1) The new meta encourages the use of multiple weapons and not just Fatebringer, Black Hammer and Gjallahorn; 2) You need weapon parts multiple times to infuse those weapons from the 280 or lower light they arrive at to 290+ to make them usable in the Nightfall, raid and Iron Banner; and 3) Because of this, nearly everyone is short on parts. The players who "use almost none" are probably running around with only 3-4 weapons that they use, which the new meta was designed to get away from. If Bungie wants us to use more than a handful of weapons, we need more parts.