Well, in OP6 it is mentioned that no additional documents which are NOT mentioned on the checklists should be taken as a positive fact to improve the quality for certain immigration case, except principal applicant work experience documents .
However, it is not forbiden to send IELTS for all your family members if you have the money and time for that. No points will be awarded for your spouse IELTS. One more thing is, that in the case of rejection, all original IELTS copies stays with CIC.
I think they should change that like Australian immigration did and award few points for spouse language proficiency.