[quitit]

I'm not a statistician so I can't possibly get into a discussion about what is worthwhile information for collection. I am aware that a lot of planning goes into such questions to avoid potential abuse (even by the government), the goal being to ask the minimum needed to provide governance while also providing a historically meaningful snapshot. I don't think there is a strong argument against having a census, it's a more privacy preserving approach to planning than other forms of data collection such as mandatory registers or combining existing government databases - medical, births/deaths/marriages, automotive, postal, taxation, education and so on (in this example the resulting database has too much information about the population, governments are mindful to keep these databases separate on purpose.)

Back to your point though: Arguably a lot of needed and seemingly benign information can still be problematic. As one commenter noted, merely indicating childrens' presence can be a problem, while such information is clearly beneficial for governance.

Ultimately protections should be in place regardless of the kinds of information that is sought, since one can't foresee all potential forms of abuse such data collection can bring, nor how such information can be merged with additional data sources to reveal more accurate profiles (as is the contemporary issue of online privacy.)