My 1520.3 is Factory Unlocked (naturally) and I'm on T-MoUS. It's not the first Factory Unlocked device I've run on their network and it's not the last one I'll run... I love the freedom of choice and flexibility of purchase options that Factory Unlocked phones allow me.
Every time I run such a device I loose access to T-MoUS WiFi Calling. It's simply a trade off I understand will have to be considered in the purchase equation. There is no T-MoUS branded 1520 for me to run and it was the only upgrade option I considered worthy of moving from a 925 to.
Would I love to have SoftCard working on my phone? You bet. Do I expect that it ever will, seeing how the debut on AT&T went? You bet I don't.
The real answer does lie with Windows Phone and Microsoft; that's where I disagree with HoosierDaddy. Google did an end-run around the carrier bullsnot. Apple rammed their system down the throats of the media, the public, the banks and the retailers and got praised for it. Windows Phone is now behind the curve and SoftCard/Isis has proven to be a fiasco due to the length of time it has taken to get to Windows Phone and the now apparent slipshod implementation.