Apparently I'm taking in more of H-Man's martial arts knowledge than I realize, because as soon as I saw this picture I knew he'd shown me similar throwing weapons during one of our discussions.
A quick search turned up concealed throwing weapons, such as
black lightning spikes,
assassin spikes, and
ninja throwing spikes.
And now, suddenly, the needlers hitting a "side" of enemies, rather than a row, makes more sense! If you're rapidly chucking darts, it's best for speed and accuracy to throw them in the same direction, rather than adjusting your aim across a row of enemies. Still, since you're not in close combat (ideally), it's feasible to hit enemies on both rows.