"The Wi-Fi calling app falls short of its potential and makes me wonder why T-Mobile bothered to include it (especially since there is no economic incentive for customers to use it)."
The intent of the app is not economical at all. It's meant to give those in a low or no signal area another option to still be able to make and receive calls. I know I would be more than happy to have it on my phone where I live.
I third that! At my house, I typically have signal strength between 2 bars and nothing. I've been using a Blackberry with UMA for nearly 2 years, and it's been awesome for that purpose. Minutes aren't really an issue for me... my family plan has 1500 and I rarely use more than 500. (Early UMA didn't even use minutes!) It is a little disappointing that WiFi calling doesn't allow hand off the way UMA did, but in reality, it was rare that I switched between WiFi and network calling areas while on the phone.