Well, actually, many users on the forum I’m on usually converse in external environments, and joining with the microphone open directly can cause ambient noise to interfere with communication. And there’s another bug: even though you click to mute the microphone on the voice page, the microphone is still actually open, and you have to toggle it off and on again to truly close it.
Compared to Jitsi, there are some differences because Jitsi has an interface that lets you choose to mute or unmute your microphone before officially joining the voice chat, but Discourse Voice doesn’t have this. I think this interface largely avoids the issue.