I like having my rear light down by the dropout - it means it shines on the road and creates a pool of light (even more effective in wet weather) which I feel is more visible than a light mounted higher up. Also makes cable routing a bit more discreet, either along the rear derailleur or brake cable, depend on which country you're in.
Problem is from a distance that light wont be seen as easily as most of it is getting thrown into a dark surfaced road, so the amount of reflected light out to the drivers is a lot less than the output of the light.
I like having my rear light down by the dropout - it means it shines on the road and creates a pool of light (even more effective in wet weather) which I feel is more visible than a light mounted higher up. Also makes cable routing a bit more discreet, either along the rear derailleur or brake cable, depend on which country you're in.