"At Messenger we are always trying to create new ways for people to communicate seamlessly with everyone," said a Facebook spokesperson. "Right now, we're testing the ability for people to easily bring all their conversations -- from SMS and Messenger -- to one place. It's a really simple way to get, see and respond to all your SMS messages in just one app. By choosing to access your SMS messages in Messenger, they're right alongside all the other enhanced features that Messenger offers."
We should note here that SMS was actually a part of Facebook Messenger before, but it was removed a few years ago due to low adoption. But perhaps with the advent of messaging apps like WhatsApp and Google's Hangouts, Facebook had a change of heart. From the Android Police screenshots, it looks like the SMS messages will be colored differently -- purple instead of blue -- and that it's an optional feature that can be enabled or disabled in settings.
Screenshot credit: Android Police
As for multiple accounts on Messenger, Facebook had this to say: "Millions of people share phones with their family and friends. Until now, there hasn't been an easy way for people to access their individual Messenger accounts from shared devices. To address this, we've launched a feature on Android to enable multiple people to log in and use Messenger from a single phone."
We're not quite sure why family and friends would want to share Messenger accounts on a single phone, but we can see this feature being more useful in a tablet, where it's more likely to be a shared household device. It could also be useful if you have both a personal and a professional account. From the looks of it, you can add accounts to the Messenger app in settings and be able to switch between them as you wish. There's no telling when these features will arrive for all Android phones, but we wouldn't be surprised if they appeared on your device in the near future.