It is the only way to ensure that there is effective representativeness of the population being surveyed. It is clear that when participation is no longer mandatory, the problem that arises with any voluntary survey comes into play: some segments of the population never participate in any survey. They drop off the radar. They disappear altogether. We can no longer report anything about these groups because the survey is not reliable or complete enough. Representatives are missing. We know that, in general, youth spend little time on surveys about themselves. The vulnerable and seniors are other such groups. Thus, there are extraordinary gaps in some surveys precisely because they are not mandatory. This cannot happen with the census.
On August 27th, 2010. See this statement in context.