Consider this situation:

1. Bug is filed against version x, bug is fixed and closed.
2. Version x ships and is eventually archived.
3. A month later, bug is reopened on a new version - it's a regression.
4. Test lead wants to update the version in the issue to point at the new version - it doesn't work!

Here's what happens: editing "Affects Version" simply adds the new version, but there is no ability to remove the old version. The only workaround is to temporarily unarchive that version so it shows up in the listbox, then edit again, deselect and save, then archive the version again.

I'm not sure what a good solution is. It's nice not having those archived versions showing up in the listbox. And if we pick a new version some people may want the archived version removed (as I do), whereas others may think it should be added.