Would it be hard to support both philosophies?
I would like to suggest a couple of configuration options that by default disallow using pip outside a virtual environment but that users with root privilege can modify by editing a config file (probably somewhere in /etc) and that would enable using pip outside a virtual environment, both as root and as regular user respectively.
I feel this would satisfy the needs of regular users to be protected against accidentally breaking their system while enabling power users to have full control of their computer and enjoy the simplicity of a single environment. Clearly this discussion suggests that debian has both types of users and we should support them both if we can.
Thank you.