I am also not so convinced that these are "legally useless documents." In my opinion, one should always assume that documents drafted in legal terms have some import. I know nothing about Canadian tort law, but I think that in the United States, were a similar document used, while it would indeed be useless to show consent, it could be proof of notice.
I think that if the JW parent decided to bring a civil suit later for wrongfully transfusing the child, this document would be admissible in that matter to show that the parent was on notice that a transfusion could be given, and potentially used to argue that a burden shifting had taken place. In other words, since the parent knew (was on notice) that the hospital would transfuse if it thought the procedure was legally justified, the burden would then be on the parent after receiving such notice to run to court and get her own court order if the parent thought it was not legally justified. I would suspect that having such proof of notice would make the hospital's legal team more comfortable with giving an unconsented transfusion.