IMO voice over data still has plenty of caveats and issues - not to mention spotty coverage means that there are plenty of areas where you'll have a <3G data connection but still have crystal clear voice reception.
Also, T-mobile offers free international data in many, many countries. This may offer a cheaper option than international voice calls for callers with traditional plans.
I have Brightspot, which is a pre-paid MVNO on T-Mobile, and it has been my experience that I cannot use my Google Voice number for voicemail like I could with a post-paid account, and I am experiencing that a lot of the short-code messaging services can't reach me, either. I haven't tried sending an SMS to a short-code service, but my suspicion is "no" on that, too.
Also, and this may be T-Mobile wide, I am experiencing that they are routing all voice traffic over the data line, which means poor coverage now results in ch-op-py dr-op-ou-t speech just like a bad SIP line would. I tried to make a call last weekend and the other end sounded just like a modem it was so bad.
Go to a store with tablets on display compatible with Verizon LTE (or borrow one from a friend). Activate a SIM on the Verizon network, data only, for the tablet. Pop the SIM into your phone and presto, data only plan that Verizon can't do anything about (Thanks to Google's involvement in the bidding process for the LTE spectrum).
You can also receive calls on your Google Voice number in Hangouts. This means that if you have data service, you can make and receive calls.
It seems the best argument for a voice and SMS plan nowadays is for traveling in areas where data coverage is poor or spotty.