Are you talking about in-game voice chat, that should be available to the game to record, or a third party tool that probably shouldn't? If the game doesn't need your mic, it shouldn't access it; if it doesn't access it, it's not part of the gameplay recording.
That doesn't mean it's "not all that useful", Linux or otherwise, just because it doesn't cover your specific use case. I can definitely see myself using it to record brief clips - on linux - without having to run OBS in the background.
I see this as a substitute for Shadowplay, which records your microphone if you enable it, which I previously used on Windows to record gameplay clips, but it doesn't exist on Linux.
Steam Game Recording can record your microphone on Windows, but they haven't bothered to make it work on Linux for whatever reason.
As currently implemented on Linux, it captures all system audio and cannot be configured to do anything otherwise, so if you're talking with friends on TeamSpeak, it'll only capture half of the fucking conversation. Making it next to useless.
I'm getting really annoyed that people are going out of their way to invalidate my opinion here.