I've done a little bit of research, and here's what I've noticed. (Just off of watching a few dozen streams, with and without mics)
(OPINION ALERT: This post contains an OPINION, not a FACT.)
People tend to lean towards streamers with microphones, because they are (sometimes) capable of interacting with the streamer, and vise versa. Just a small little observation, and is not 100% supported. Just something I managed to notice while watching a few streams myself.