Honestly, it looks cool (I installed it yesterday) but like you said, no one's using it! And I've already got Skype and Hangouts installed on my phone (not to mention the built-in phone and SMS functionality of my phone - but I guess Wire, like Skype and Hangouts, works from a tablet too).
I suppose, though I doubt it will ever happen, if everyone used SIP & RTSP then they could concentrate on the user experience and users could choose the experience they prefer without having to choose a network too. But as long as the services are proprietary the incumbent will always dominate.