The only real benefit from 'buy backs' is the removal of a firearm from a place where it is unwanted and/or can't be stored properly. Once the gun is handed in, that benefit has fully accrued.

Firearms are worth money, and it's a very appropriate use of this resource to convert them into funds for public services. Destroying them provides no public benefit (and may even have a cost); firearms are fungible, and destroying the tiny number of them passing through buy-backs does nothing at all to deny a firearm to anyone who might want one...