The "mod_userdir" fuctionality has some security risks associated with it. As you can see by the documentation, it is only used when a visitor accesses their website via a username.
For example: http://example.net/~username
Where username would be the username of the website user. This is a very niche option, so I don't believe that you need it unless that is how customers get to your sites.
AbanteCart Admins access their admin panel with the user name in the url . In discussing this with my server manager he "highly suggest discussing this with your developers as they are likely more familiar with the way Apache is set to handle logins to the site. You will likely find the directives to do this within your .htaccess file."
so my question remains unanswered - to enable userDir or not?