Support for updating a belongs to association from the foreign key (without saving and reloading the record)

Hi all,

I would like to solicit feedback for this ticket:
http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/142-patch-belongs_to-association-not-updated-when-assigning-to-foreign-key.
The reasoning is explained in the ticket so I won't repeat that here
but I would like to hear any opinions on why this should or should not
be applied.

The latest patch (as of today) is available here:
http://rails.lighthouseapp.com/attachments/44268/belongs_to_update_association_from_foreign_key.diff

Cheers,
Jon