It seems the MemberDirectory module will strip out the Username in the results if the requesting user is NOT an administrator.
It seems this is "by design" and will not be fixed "for security reasons".
I feel the use of Usernames should be up to the site administrator. For security purposes - sure it can be turned off by default - but give the admin the option to use it. This is how every other user identifying profile property is setup. Usernames are also likely "web alter-ego's".
EG: A beer enthusiast community. There could be multiple Andrew Walker. And if these multiple Andrew Walker's were posting to the same group journal - there would be confusion as to which was which. Especially if both like to use various ridiculous cat photos for their avatars. And avatars can change frequently. By displaying Username - it allows the COMMUNITY to identify their members easily.
Also - I would expect most of these community members would DIS-like their email being posted to identify them - yet the MemberDirectory Module will return the email address.
I appreciate the effort and initial judgement for security reasons - but feel it's over-reaching and should be toned down to be an Administrator setting.