For Family Class sponsorship, the spousal/common-law/conjugal category is actually the most common category of sponsorship, and of that, I would venture to say that spousal is most common. Most of the time in these cases, there is only one principal applicant.
I can totally see how this can be confusing, but when they say "his or her spouse or common law partner, if applicable," they are talking about the spouse of the principal applicant when that spouse is NOT the sponsor (see here,
http://www.cic.gc.ca/english/information/applications/fc.asp#immigrate, it's in Part 2: Immigration Forms, and all of those are filled out by the individual/s being sponsored, whereas those in Part 1 are filled out by the sponsor).