Ashley - the following scenarios should more effectively demonstrate what I mean. I also found out some other things as well.
Case:1 You want a situation where multiple lights are used. Because shadows draw instead of light you get an additive effect with the shadows. All the shadows draw despite that they are lit by another light. Using opacity to control the shadows doesn't really fix the problem because the shadows still build up even where they shouldn't. If light was drawn then you can use it to cut out of a black filled layer resulting in light falling how we expect it to.
SubCase 1a: This scenario also brings up another problem that indicates an additional feature is needed to the light and shadow behavior/attribute . Light fade distance and cutoff. You can easily do this at the moment by creating the black layer and using a sphere to stamp out a light circle. You then draw the shadows in and presto, you have a really neat area of light complete with awesome shadows. But then you want another light, and the whole model is broken. all of a sudden dark shadows cut across fields of light that they shouldn't. You can't even fix this by adding more layer.
Case 2: When using a layer to render light effects, including illumination areas and so on, the light layer usually has to be above, the tiles needing to be lit. A problem arises here in which the block that is casting a shadow actually gets covered by that shadow. You can't add in those blocks on the layer above light because then they will be just as light even in "dark areas". If light was drawn, you could draw it as black and then use draw the shadow casters as black on the same layer. Finally a white sphere sprite could be put on top of it all with appropriate blend mode to not cover any shadow area.
Currently, the shadow being drawn where the shadow caster is would be nice to toggle but not needed if you can draw light in stead of shadow.
In my mind, the addition of the light and shadow behavior is super awesome as it adds in a slew of professional effects to construct. It's very exciting. But adding in the ability to draw either shadow or light would make the system more robust and seems like an essential feature to me. It would really add to its flexibility. Other features such as light fade (linear fade at the least) and light effect distance would be really, really nice. I know you can do this via a build up of layers and sprites but wouldn't it be more cost effective to have it all together in terms of performance? Also it is imposible to have multiple lights behave as expected because of how you have to use layers to build them up. Including light distance/fade would solve this. Lastly, being able to reduce the arc that light shines would be cool to, but would be much more of a bell and whistle. Essentially being able to set the angle and radius in degrees of the light (for easily creating beams of light).
In conclusion I feel that the following options would really help create a strong light/shadow model thus increasing the behavior's value to the construct community. (in order of priority/need in my mind)
1. Draw either shadow or light (needed for flexibility and certain situations)
2. Light fade distance (Fade type (linear/expo/none), distance(in pixels) (needed for multiple lights with area of effect)
3. Angle of light (from 0-360) (for creating narrow beams of light)
I hope you will consider at least the first one (and the second would be super nice too), I shouldn't expect it would be too hard to implement and I think it is necessary. The list of found cases where drawing light would be better than shadow will only grow as people have time to work with it.