Back in 2010, the Canadian government opted to make the long form portion of its 2011 census voluntary. Researchers who use the data in their work, and policymakers who use the data to make decisions were concerned about how a voluntary survey would impact the resulting data.
As expected, the early analysis suggests that the lower quality data will lead to higher spending. Those costs might not be borne by the national government, but the provinces, local authorities and other parties that have used this data to track changes in their jurisdictions. Without this data, they must pay to replace it, and due to the lower quality, they are paying more for less. Smaller jurisdictions have been harder hit from this change, as response rates have been lower in rural jurisdictions, and smaller governments are less likely to have the resources to fill those data gaps.
The Canadian Chamber of Commerce has called for the long form to be mandatory in the next census (2016), and a bill was introduced in Parliament to that effect last week. With the Conservatives still in majority, it seems likely to fail. Those in the United States may not find themselves concerned, as we still have 5 years to the next census. However, the Census Bureau administers the American Community Survey, and there have been efforts to curtail that in the past. It may happen again.