At 4.5, would you map a domain to your site definition and access the instance with, you'll get your site being rendered at PageEditor with all the green bars for you to start with content editing.

Would you configure the same at 5.x, at the moment the PageEditor will not be featuring the green bars for you, from the user perspective you basically end up with a static page without any option to do anything else there other than requesting the appropriate URI pointing back to UI.

A login form redirecting to UI could eventually be used would users prefer to always end up with applauncher for instance, e.g.

<form id="loginForm" action="${contextPath}/.magnolia/admincentral" method="post" class="mgnl-form">