Linear electron-electron colliders

In the framework of the discussion on what shape our future machine arsenal should take so as to maximize our chances of penetrating beyond the realm where our astonishingly successful Standard Model of Particle Interactions holds undisputed sway, the present contribution is somewhat unusual: I am not here to convince our community to build yet another machine. Instead, my task is to convince you that in the established choices that we are headed towards, it is of great importance that the Electron Collider of the next generation, i.e., in the 0.5 to 1.5 TeV energy range, should be configured just such, as an Electron collider, NOT dedicated to just one incoming charge state (say, e+e−).