In what world is adding a custom field to a bug tracker and maintaining it cheaper than anything? If someone proves out this workflow and releases a bug tracker that has the functionality built in then I'll consider adopting it, but I'm certainly not going to be the first.
Now when considering priorities, consider not just impact and age, but also times checked.
It's less expensive than going and deleting stuff. Unless you're automating deletions? In which case... I don't think I can continue this discussion.