Okay, I'm pretty sure that people find it tedious to have to pull a weapon from the Vault just to delete it. Especially when your slots of a certain inventory are full. Run to the Vault put something away, pull something else out, back out of the Vault, delete, and rinse/repeat. An even bigger pain is doing it via the Postmaster. Drop X amount of weapons in the vault to pick up X amount of engrams, decode, delete, than go back to the vault and get your stuff again. I feel as though being able to delete things from the Vault or from the Postmaster box would make trips to the tower run more smoothly.