> Can the Photon be left in this "signalling" mode?

Yes.

I suspect the Photon Freedoms switches vary in their stiffness. Some switch more easily than others. If you think yours is likely to latch itself on by a momentary pressure in your pocket, you can leave it in "signalling mode" so it goes off again when the pressure is released, and doesn't run the battery down.. I think this kind of thing, with the dimming, makes the circuitry worth having despite the loss of simplicity.