When profiles are enabled globally, they’re public by default — i.e., you have to opt out, rather than opt in. For this reason, most NationBuilder customers I’ve worked with have asked that public profiles be disabled.
It seems unnecessary to require public profiles to enable directory pages. When a user configures their account settings, they’re able edit their “public profile” bio whether or not public profiles are displayed on the site — so there ought to be no conflict on that front.
A nation should not be required to display a public profile of every supporter in its database in order to use the otherwise incredibly powerful directory page type. It seems akin to requiring public profiles in order to use the Social Slider.