Modern web UI is nice, but at the end of the day when it comes to mobile, apps are still king.
If the purpose of this is to replace the interaction we had on Yammer, a mobile app is a necessity...
One very important aspect of mobile apps compared to browser view is authentication. With mobile apps you can usually save your credentials for quick access to content.
In my case I'd have to fireup my password manager everytime to view stuff in my mobile browser.