This is one reason I'm excited about the Google-Dish wireless carrier news. If they made a 4G-only carrier, you could use VoLTE for calls. By removing the need for non-4G chips, phones would become lighter, smaller, cheaper, and less hot when running. And hopefully service would be cheaper and more stable too.
Doesn't seem to make a noticeable impact on most recent VZ phones, and as far as I'm aware VZ doesn't run VoLTE[0] yet.
[0] Anecdotally, I'm hoping that's right. I've been on (GSM/UMTS) T-Mobile for years and switched to VZ a few months ago; and I've noticed a marked drop in call quality & stability vs. every GSM phone I've owned.