Unfortunately, most people who don't declare family members do so to "hide" the fact that their family member is inadmissible to Canada for some reason (usually criminal or medical) because they know that inadmissibility will make them inadmissible also.
From what I have seen on messageboards of people who this has happened to, it doesn't seem to me that most of them are trying to hide anything, rather they did it because they didn't think it would matter and they didn't want to delay their own PR and really thought it would not be a problem sponsoring their spouse later.
However, the CIC rules are clear. In the skilled worker application guide, it says:
You must list all family members, whether they wish to be considered in your application for permanent residence at this time or not. You will not be able to sponsor family members at a later date if they are not listed on your application.
In the operational manual for skilled workers, it says:
Family members can be added to the application at any time during the process, including after the visa is issued but prior to obtaining permanent resident status. Applicants should be counselled to inform the visa office immediately if their family composition has changed.
And in the operational manual for family class, it says:
Applicants must declare all family members when applying for a visa and must again declare all family members, whether accompanying or not, prior to obtaining permanent resident status. Permanent residents who did not declare all their family members on their application are reportable under A44(1) [see also “Sponsor who may be subject to an A44(1) report” (section 10.5 below) and “Misrepresentation” (section 5.22 below)].
And in 5.22 Misrepresentation on page 18, it says:
A foreign national is inadmissible for two years for withholding or misrepresenting information that is material to making a decision on an application [A40].
And in section 10.5 on page 43, it says:
Sponsors, who became permanent residents on the basis of never having been married, sometimes attempt to sponsor spouses they married before immigrating. Similarly, sponsors who became permanent residents as single children might sponsor a common-law partner with whom they lived in a conjugal relationship before immigrating. In such cases the sponsor may be reported under A44(1) for misrepresenting a material fact [A40(1)(a)]. In addition, the original sponsor of a family member who misrepresented their marital status may be reported under A44(1).
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