It's not about what's best for the customers, it's about the rights of the company to that list, and their choice to make it public or private. Privacy is a right, not a privilege, and requires no justification.

Yes. Clearly. Competition benefits customers, and better market information allows better competition.

Would it be good for you? Yes, probably, but it'd be scary. If your competitor can take your customers from you then you need to improve your offering. If they can't then you can probably take their customers from them.

Who benefits from keeping your customer list secret?

It sounds like you're saying that the ends justify the means.