Perhaps, when the option for ‘Spatial Music’ is set as disabled in the ‘room’ settings of a stereo pair.

100% agree that a balance option is needed. 

Just to expand slightly on my own thinking here…

Because of all the front/left/right & upward angled drivers in each Era-300 speaker and their overall global immersive effect when ‘paired’ in a room playing Atmos audio, my personal thoughts are that the Sonos engineers may have felt the audio was less sensitive towards an off-axis listening position compared to other general ‘stereo only’ speakers, making a balance control somewhat unnecessary, generally speaking.

The soundstage is certainly broader with an Era 300 pair …and perhaps that overall, makes the sweet-spot much wider too, but I think switching over to stereo, by turning off ‘Spatial Music’ might (perhaps) be a possible reason to introduce a stereo left/right balance slider control, but there are undoubtedly arguments both for and against it. The Era 100’s with their angled tweeters, currently have a balance control in their room settings for example, so that’s one argument for the control.

I do wonder though, if some users might forget that they adjusted/disabled such ‘Spatial Music’ settings, which may give rise to a secondary issue of them asking why their speakers are not playing Atmos audio and that in turn could lead to unwarranted calls to the Sonos Support desk. So maybe that’s why there isn’t a balance slider control at this present moment in time?