It would be a useful tool to be able to turn on a mirror (particularly in the x axis, but choice of x, y, and z is nice)

This would be amazing specifically

Yes, that could be cool

Yes Please!  

You mean like the tool that's already in there?

+1. Maybe something simple like Brutus Simetry  on photoshop or the simmetry function in  autodesk sketchbook

Actually an easy way would be  set a plane, and then use the same button for mirror.

You mean like the 'M' key does while having an object selected? The feature to mirror objects on certain axis already exists; make sure to have the correct axis enables in de top of the screen, when pressing 'M' all selected models should het mirrorer accordingly.