But sunlight reflecting off a random planet (including ours) is not useful. Sure, you can detect it, but it doesn't say anything about whether the planet has intelligent life or not. It would just be noise in the cosmos, no signal.
I'm not a physicist, but my understanding is that all EM frequencies -- visible light, radio, infrared, etc -- travel away from earth at the same speed (c). Since space has (almost) nothing to slow them down, then they'll just keep going forever. Thus, "strength" (however you want to measure it) is largely irrelevant so long as the signal it produces is detectable apart from all the other outgoing information. So the "light" broadcast from a TV tower is the same as the light from a spotlight at a shopping mall, in terms of its ability to travel through space and get noticed by aliens.
The only exception I can think of would be our atmospheric interference. Some light may penetrate our own cloud layers better than others. So all light may not be created equal, in that sense.
