IMHO, I believe that tickets fall under a different category than invoices or transactions, which cannot be deleted. We have multiple departments set up that anyone can email in to, and many of them are spammed constantly. Generally we just close the ticket, but an option to purge the ticket would be nice. We have no plans on deleting client or prospective client tickets, but if we want to review our support tickets, we have to sort through the spam to see tickets that really mean something.
As a temporary workaround, we could create a 'Trash' department that no one is assigned to, and transfer all the junk tickets there, but an actual delete option would be really nice.