If auto-fill is enabled, it will blindly write into matching fields on a page. This includes fields that are already populated, which can cause serious problems.
As a web site administrator I can edit any user’s account. If I open the edit page for a user, auto-fill replaces the user’s email address with mine, and writes my password into the password-change fields. I will have opened the page to change something else, definitely not these fields. This means I cannot use the auto-fill feature at all, even though for most sites it’s very convenient.
Auto-fill should only ever fill empty fields, fields with content should be left alone. Furthermore, I would prefer that if any candidate field on a page is not empty, then none of the fields should be filled.
Another option could be borrowed from LastPass, which is to only auto-fill pages that are launched directly by BW. Having this as a user-selectable mode of operation would be very good.