Since bound forms have data attached to it, they will also be validated by the usual methods. If we add the tags to the list *_errors* again, *BaseForm.is_valid()* will never return *true* even if all fields were validated successfully. On the contrary, *BaseForm.full_clean()* ignores unbound forms which is good for the purpose of template rendering.
This seemed like a fairly safe way of making that distinction without having to have two classes for each form - one with and one without the mixin - for server-side validation and rendering.
Anyhow, with the changes we discussed this is likely obsolete.