I think it could be useful to collect this data, both for administrative and research purposes.
I’m unsure, currently, exactly what data should be collected by the censuses (that would be proposed and discussed here). The data that is collected, should be collected anonymously. Furthermore, participation should be entirely voluntary.
I can see how users would be uncomfortable with the idea, but as long as it were voluntary and anonymous, I don’t see the harm. I remember enjoying looking through the Lemmy.ca stats which Otter compiled.