As I said earlier, there are some needs that users have out there for which you need a census. There are other needs that can be satisfied quite easily by a voluntary survey.
The questions that are in the census are, in my view, the questions that can be answered...or most of them are. Maybe there is the odd one here or there that doesn't belong there, but in terms of the long form overall, you're not going to get the reliable information you seek on those questions from a voluntary survey.