You always need a reason for collecting data
When you ask for data from someone (member, customer, prospect, etc.) there has to be a clear reason the person you're asking will provide that data. (The reason needs to be clear to the consumer!)
If you're selling a membership or event registration, asking for contact info, including email address, seems pretty self-explanatory. After all, as the consumer, I understand you need that information in order to communicate with me.
But when you start asking lots of other questions, like my age, or ethnicity, or highest level of work experience, as the consumer I start wondering "What are you going to do with this data?" And the more the consumer starts wondering what you're asking, the less likely you are to get a response. Or put another way, asking for more data is likely to lower your response rates (i.e., fewer buyers).
So it is incumbent upon us to always explain why data is being collected if it isn't absolutely clear. Otherwise the consumer will vote with her feet.