In places such as "What's happening with my network?" web part and tag profile pages, User Profile Service presents a set ...

In places such as "What's happening with my network?" web part and tag profile pages, User Profile Service presents a set of URLs to end users. These URLs are trimmed for security to make sure that a user does not see  URL to which he does not have access to. User Profile Service uses Search Service to perform this trimming of URLs for security. If there is no Search Service associated with the User Profile Service, this trimming of URLs will not work, and hence all URLs will be visible to everyone. Of course, when a user clicks on a URL to which he does not have access, he will still be denied access.