Is it always a best practice to decouple the admin area from the frontend portion in a WordPress application? Are there any instances when one might want to use the admin area/screen as the application's main interface?
Off the top of my head I can't think of any instance where you would want the WordPress admin to be the main entry point for your website, that there isn't a more suitable software to use. Do you have a specific application or use in mind?
In general I would say say, yes it is best practice to keep the admin potion of your WordPress install for "Administrators" as it could be used to upload many forms of malicious or abusive code.