If we were standing in bright sunlight, I would expect the light from a small bulb to be barely visible. From a distance it'd be impossible to tell whether the light were on or off, and thus a lack of light would not immediately give away the illusion.Nail said:
Now, I point the flashlight directly at you; you now see the end I've painted to appear as if it's on. (a bright, cheery yellow, I might add) You're still quite a distance away from me, in a brightly lit room. Would you ever be fooled into thinking it was on?
The same goes for a torch, to a lesser extent. If you've ever watched a campfire during the daytime, you know that the flames are translucent and rather dim, and not easy to see. A character seeing an illusionary torch, in an otherwise bright area, could potentially be fooled by translucent images of a flame.
Anyway, you seem to be going at the point backward. You claim that figments must be able to shed light, because otherwise the illusion of a light source wouldn't work. Even granting that assertion, the rule is that figments cannot shed light; if that means an illusionary light source is unbelievable, then fine, so be it. In a medieval setting, the only things that shed light are fire and magic, but you can still make illusions of everything else on the planet.