Because point lights are not UV mapped, which means the shader has no angular data at all. It would have to compute that, which I can do, but it requires that every time you move the light, you type in its new coordinates into the shader.

If there was a LightPosition node, this would not be a problem.

But then you have an additional problem - you can't rotate a point light. So if a directional light is implemented as a point light pretending to be a directional light, you have a hard time controlling it. You'd have to enter rotation parameters into the shader to change its orientation. This would be very tedious. Entering coordinates every time you move it, and rotations every time you want to re-orient it. That's possible but not usable.

On the other hand, a pair of spotlights, with one parented to the other, would work great.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)