Wi-Fi calling is going to become a lot more important with the proliferation of Wi-Fi hotspots all over the world, which I believe will also ultimately disrupt the carriers. But until we get there, carriers are actually going to support it so they can tout one extra cool feature that they have and the competition doesn't. It all ends up being towards our benefit.
T-mobile will put their Smart Wi-Fi calling app inside myTouch and Motorola DEFY, for now, with other devices to be supported later. Is this a big deal? I think it is, but not directly a great feature for consumers. It's good because it gets the other carriers to accept Wi-Fi calling, but when it comes to the consumer, right now it's a much better idea to just get Tango or Skype apps. Both support Wi-Fi calling, and they work on all Android 2.1+ phones and iPhones.
Why would you use T-mobile's wi-fi calling feature that enables you to call only certain other devices (only 2 of them), when you can use the other apps to be able to call a lot more.
Ultimately, these apps will become irrelevant in the long run as Google will most likely implement wi-fi calling, VOIP through 3G, and video-calling, by default in the next iterations of Android - perhaps as soon as Android Gingerbread.