Privacy

They provide a suite of services, most of which can be provided in a private manner. Blowing a hole in that by providing email seems counter productive. As I said, they could point you at a separate email service. Even if they provided that service it could ensure an adaquate break between the private services and and the non-private.

As a service, is it more privacy conscious than, say, Gmail? Yes, but you're still ultimately just asking the postman not to read your postcards.