Remote Desktop will only allow you to control and observe a Mac if the ARD user has a blank password. All other tasks will not work.
The status will show "No Password" and will not appear in a 'ready' status.
To perform other tasks using Remote Desktop, specify a user who has a password configured when adding the computer in Remote Desktop Admin.
You may also use Directory-based authentication in this scenario. Refer to page 63 of the Remote Desktop 3.1 Admin Guide for more information about directory-based authentication.