I was just wondering why he even sells them still.

In case your a crazy collector? There's no real reason at all for the moment, especially since Sparrows are non-essential, and you can get a free SRL type sparrow from either playing the April Update Content or from Sterling packages. Or if your really lucky, SRL packages from the Nightfalls. Or solo Crota's end till you get a raid sparrow. They could easily give the plasma drives a purpose again just by making 'em work on the new sparrows!