I guess it depends on the circumstances of your using it. TM provide the feature for a couple of reasons:
a) As an alternative means of calling for customers who have wireless signal issues, normally caused by building structures, since there is a reasonable chance of having wifi service available in such locations.
b) As part of a longer term strategy of offloading wireless network traffic to maximize subscriptions without overwhelming the infrastructure.
Neither they nor any other carrier are going to be motivated to provide a free alternative to customers paying for plan minutes just because the technology enables it to be done - particularly as there are costs for TM in delivering the service.
Having said that, TM are allowing subscribers to many of their more recent plans to activate the service without it depleting their pool of minutes under their plan. Thus, anyone covered by condition (a), whose calling experience is improved, will not consider the feature useless, and anyone who is able to get the "free wifi calling" plan addition will not consider it useless.
As for your perception of what is and is not "wifi calling": unless you are doing point-to-point calling over the internet (such as Skype), where you are not using a conventional phone number to dial, what you believe to be "calling over the internet" is actually doing pretty much what TM are doing here, but the provider is simply using a different funding model to provide the service.
In other words, if you use a computer-based client to dial a conventional phone number, the call is being routed over the internet as far as the provider's Internet network presence extends, but it is then being routed out to the Public Switched Telephone Network to complete the call, just as it would do if the call originated from your home or office phone, or a normal wireless call.
it doesn't cost me a dime anyways im unlimited. but i still don't see the point in it
Where I live, I have a great signal all the time. However, I work in an office building on the edge of whatever tower is around here's range, so depending where I stand in the office, I either get one bar or no service. T-Mobile's Wi-Fi Calling app remedies this by connecting to the T-Mobile network over the office's wireless internet rather than the cell tower which is barely in range. I'm able to place and recieve calls even though I have no cell coverage. Out of all the proprietary bloat T-Mobile crammed onto the Wildfire S, this is a great app.
If you have a great signal 100% of the time, then that's cool, just turn off Wi-Fi calling and don't worry about it, as you don't need it. It consumes plan minutes because you're still talking on T-Mobile's network, even if it's over the internet. It's not much different of a concept from Vonage or Skype.