Not because I to see them while I chat with them, but because the simplicity is there and it's enough to make me think twice before opening either app.
Once you've made a couple of calls, the bottom overlay will start populating with your most recent contacts. (You can't see me now, but you will in a while.) If you're a geek like me, the first thing you'll look for are the settings.
Duo's calling interface only shows a couple of buttons overlaid in circles on top of each other on the bottom left: And there's obviously a red button to end the call in the middle bottom of the screen.
If you're on the receiving end of a Duo call, you'll see the recipient before picking up if you both have Knock Knock enabled, and Duo will also show you which network it's currently using for its call.
Audio-only or group video calls aren't yet possible, there's no web or desktop client, no multi-device support, and no way to share what's on your screen for example instead of what you see with your cameras.
But even though these seemed like deal-breakers to me when the app was announced, after using it for a few days, I'd argue that they're secondary features to what Duo is trying to be: an instantaneous way for you to get in touch with someone else as if you were near them.