Some sound cards have two or more line outs. If you use more than one line out for sound, it would be handy to be able to control volume of all of them by single plugin object. Also muting/unmuting should be applied to all of the tracks by one middle-click. E.g. for Creative Sound Blaster Live! Value sound card the 2nd line out is controlled by "Wave Surround" in some environment (e.g. Xubuntu 8.10). For some reason I can't mute it by "amixer set 'Wave Surround' mute" but "amixer set 'Wave Surround' 0%" mutes it effectively. In case you want to control playback by combination of 'Master' and 'Wave Surround', muting could be done so that 'Master' can be muted properly (like "amixer set 'Master' mute" command), but level of 'Wave Surround' should be set to 0% and back to same as level of 'Master' when unmuting. Generally, levels of the selected tracks should be kept identical when not muted.
User can set different mixers to different levels anyway by e.g. alsamixer, so in my opinion it is better to forget keeping many line outs at same levels by the Mixer plugin. Usually using software mixers of applications are better way to adjust volume level anyway. Still the Mixer plugin can be good in that it gives visual information about the state of overall playback (at least about one mixer track per plugin) and capture mixer settings, and it gives a quick way to have some control over those settings. Maybe the easiest way give user a fast way to toggle capture/mute would be to add two input fields in Mixer Plugin properties: optional command to run after mute (or nocapture) and optional command to run after unmute (or capture). That way e.g. proper muting can be done by one (middle) click of mouse.
Actually you can toggle all outputs you need like this even with the current version: add a custom left-click command that toggles wanted mixer tracks. Then use left-click instead of middle-click to mute/unmute. Why not just make middle-click command configurable, too? (It could have a toggle command as default.)
This was unclear enhancement request with the comments.