I believe it will weaken the concept of steem, where everything is out in the open. It is my opinion that the hidden groups on facebook, creating members only places, is one of the reasons it has become a sour and jealous place.
I do really like your app and believe you must have tought this through well.
Why do you think it will weaken concept of Steem?! Private messages are being used already through different channels (discord, steemit.chat, etc.) It is not encouraging creating members only group or hidden groups that is what upcoming Steem community feature is for.
I think the pm feature will make things easier :)