That is an old trick. Those buttons you dropped in an Edison socket that they used to sell to extend bulb life were diodes. Logic would say they reduce brightness by half but it actually works out a little better than that but it does seem to make the bulbs last longer. I have heard theories that it is the DC aspect but nobody has convinced me.
In the case of LEDs it would have virtually no effect if you had it forward biased along with the LED. You would have another .7 voltage drop but that is pretty insignificant at line voltage.
