My opinion is based on the body of research carried out. I do a lot of quantitative analysis. The response rate for surveys we use and administer in ordinary conditions is steadily declining. A few years ago, approximately 75% of the population answered questions for an ordinary survey. Today, a response rate of 50% is considered to be good. And yet, it is clear that an entire segment of the population is underrepresented. This cannot happen with the census.
On August 27th, 2010. See this statement in context.