I haven't done too much research but I've heard that data plan only without voice plan costs $50, not $30.
We do need stable data network and good-quality voip provider if and only if you make call and receive calls via VOIP. Yes, verizon network is much better than ATT, but there could be traffic jam or data channel loss sometime somewhere which I often experience (3G downgraded to 1x). If you do not use voice (either or not voip) at all, ok, just texting, but you will feel that people do need voice for communciation eventually
Furthermore, Google Voice text messaging seems not as stable as traditional SMS. And GV may start to charge user next year.
You need to think of lots of factors.
