I agree with Wally 100%.
If you're using the information to do your job, there should be no issue. If you're planning on using it for marketing purposes, then you need to first convince your customer that you're an expert - you're the person they NEED their answers from. At that point, they should be more than willing to give you an email address at the very least.
I used to work for a retail company that sold imports. Every single customer that came up to the counter was asked for their name, right off the bat. Why? Because we had the names stored in the computers, and could easily help manage their purchases for them should they ever have a problem in the future and had misplaced a receipt or something like that. No, not every client gave us the info, but by explaining it to them, the majority had no problem giving their name, mailing address, phone, email, and so on.
Sure, the company used that info to send out announcements of new products, catalogs, etc. But they also knew that if there were ever an issue with something, simply flashing someone their driver's license would help assist them in solving the problem.