If by "who cares" you mean its absence won't keep a savvy user from communicating, you are right. But people who just know how to place and receive regular calls care.
Shoot, I'm a savvy user--and you know what? I just don't WANT to jump through a bunch of hoops, or make people calling me jump through hoops, just to hold a voice conversation. I'm old enough and I've earned that right.
Over the last two years I've spent quite a bit of time visiting family in the hospital. The hospital has free wifi, but as you can imagine it does not allow much if any T-Mobile cellular service through. And not once did I care, because T-Mobile wifi calling just took care of it without my having to think about it ***as I visited family in the hospital***.
For example: My wife had knee replacement surgery, then a year later had that revised (thanks, Zimmer, I can't wait for your check for THAT screwup), then a few months later had the second knee done...and for each of those instances, I had to wait for status update calls from the staff. That's right, the medical industry DOES NOT use email. Phone calls is it. The hospital is the LAST place I want to be fiddling with things just to be able to make a call--and certainly I couldn't ask them to fiddle around just to call me. T-Mobile wifi calling to the rescue. It. Just. Works.
And yesterday T-Mobile upped the ante with double the data (I'll be up to 18GB of high speed data at that point, with talk and text the price will remain at $75) AND Netflix streaming not counting against my data. Add everything up and it's an attractive package.
Wifi calling is a strong driver for me. Back on topic: I'm closely watching the Blackberry Priv, and I even took a look at one today at the AT&T store. It's nice. Is it $700 nice? Well, as a Lumia 635 user I'm anxiously awaiting the 950 family--the XL in particular--because I like Windows Phone 8.1 and I fully expect to like Windows 10 Mobile and the 950XL hardware. BUT: if they've screwed up and if putting a T-Mobile SIM into it does NOT give me full-on T-Mobiile wifi calling and VoLTE over band 12 (and mobile hotspot), like the Blackberry does, I will dismiss the 950XL as not useful to me and I will go buy the Blackberry at $700.
Because I have standards and requirements for my phone that go well beyond a pretty face.
I really, really want the 950 family to succeed. Here's a reason I don't want the Blackberry and Android: Google Now can't figure out my name. Seriously. On my wife's Galaxy S4, brand new from Cricket, if I ask it "OK Google, call (my dad's name)" it simply gets confused. Cortana on Windows Phone 8.1, on the other hand, immediately recognizes my family name and happily dials my wife, my father, my brothers, whatever. So Cortana is better than Google Now in a way that's important to me. I fully expect the Windows 10 version to be even better. But only time will tell.