Update (23 September 2016): Looks like Google Assistant just confirmed that Allo will support SMS in near future:

Thinking of switching to Google Allo for all your messaging needs? Well, you can’t. While messaging experience on Google Allo is rich in many ways, but the app doesn’t feature SMS fallback support which many of you wished for.

There’s a heated discussion going over at reddit regarding SMS fallback support on Allo. In case you’re lost on the topic, here’s a quick wrap-up from user rgrasell:

The whole point of SMS fallback is to communicate with devices that don’t have your specific app. Even if Allo only came preinstalled on Nexuses, SMS fallback means you can communicate well with other Android and iPhone users.

We believe Google will push SMS fallback support to Allo at some point in future, or maybe with the new Pixel phones coming on Oct. 4.

Google Allo SMS Chat

That said, you can still send messages from Google Allo to people who don’t have the app installed on their Android phones. And some of them will be able to reply as well (without installing Allo), Google has a system in place for that but not all Android devices will support it so everyone may not be able to reply unless they install Allo.

We’ll be sure to update this post as and when Google releases SMS fallback support for Allo.

Happy Androiding!