People want enhancement cores to be removed from infusion, which I agree with. But Bungie, though, wants to make infusion "meaningful". People also want to get rid of baryon boughs because they merely take up space in their inventories, with numbers in the 10,000s. A solution to these problems is to have the Spider sell enhancement cores/legendary shards for baryon boughs. Bungie gets its wish to make infusion meaningful, players no longer have a headache over cores, and they free up space in their inventories. From a lore perspective, this makes perfect sense. The Spider wants to get materials from the Dreaming City but can't get into it, so he sells what players want, enhancement cores, for what he wants, baryon boughs.