As already pointed out, it will use alot of energy
One problem with all tail lighting schemes is that the typical motorist has headlights on, they will light up your rear reflector and other stuff on the bike, no tail light is needed. A rear tail light only makes sense if the driver is incompetent enough to leave the lights off, but still able enough to see your blinking whatever. I see little evidence that distracted drivers will see anything you hang off your bike.
BTW, I do own and use a rear blinky. I doubt it does much.