The issue with this form, and others, is that they are not specific to just spousal sponsorship. They are used for every immigration application, so CIC has made them, and the accompanying sections in the guides, fairly generic. Immigration streams like parent/grandparent sponsorship, CEC, FSW and PNP often have both the principal applicant and their spouse immigrating; as such, both must fill out those forms.
The distinction for spousal sponsorship is made in the guide, under "Who should be included in this application?"
http://www.cic.gc.ca/english/information/applications/guides/3999Etoc.asp#family
If you are being sponsored as a member of the family class, your spouse or common-law partner (except where your spouse or common-law partner is the sponsor) must be included in your application as a family member.
That means that for the purposes of spousal sponsorship, the sponsor is not included in the app as a family member and therefore doesn't need to fill out those forms. CIC could definitely explain it much better than they do but as we all know, CIC is not known for clarity...
Thank you Canuck_in_UK for clarifyingthe issue. I hope someone at CIC reads what you say and revises the instructions for filling out IMM 5406 in the light of it.