There is a confusion when it comes to nicknames in group chat. In corporate environment usually real names should be used in all aspects of IM. Admin can set display name for a user in Admin Console or pull it via LDAP and then it is shown in the roster via shared groups. Btw, Spark still defaults to username when adding a contact manually, which is also confusing and not convenient (linked ticket).
So, maybe Spark could pull user's display name from the server and set the nickname field in the profile to the same value. It should also automatically update when display name is changed on the server. As some users might prefer to use different nickname in group chats, this could be a checkbox (maybe even in profile window below all the fields or in Preferences > Group chat). This could be enabled by default as most users would want to use their display name in group chat.