I have no contract smartphones and haven't paid for service in years. This is a great way to go that you are suggesting and it has worked flawlessly for me. I don't do much calling anyways so why get stuck into a contract. If I have to have connection without wifi, I have a no contract hotspot for peanuts compared to a smartphone on a contract from a provider.
Talk atone is awesome! Way more useful then the regular google voice app. I Wish there was a way to remove adds with out subscribing monthly. Which is lame. Other wise very happy with it.
If you know other people with iOS 7 you could use Facetime Voice, to make voice only calls over Facetime. This turns the device into a phone, but only for people with Apple device using friends.