When using a light sensor the output is equal to the light level it detects. This however causes it to always emit a signal, which will travel very far (>15 blocks). This renders the gate effectively useless, because you either have to add a complicated set of logic gates to diminish the signal or you need to have a long line of wires so the signal diminishes on its own.

I think it might be better if this has a binary signal (with monster spawn level as threshold), either as default or as one of its settings.

Light sensor is analog, gates that are going to be implemented soon(tm) will help with this.