I think the evidence is in fact that people such as these, who are somewhat marginalized, would be less likely to fill out the census form. I would agree with you that they will therefore be underrepresented in a voluntary census. When it is mandatory, I think they realize, despite the struggle they have and despite their reluctance, that it is important, and as a Canadian citizen they feel they want to contribute to it. That signal of “mandatory” is sent to them, and as a result they fill it out, despite their reluctance. Without the mandatory aspect, they may feel, well, let someone else fill it out.
On November 23rd, 2010. See this statement in context.