Agreed on the lack of signaling by many drivers. Still, I think the mirror signal feature would occassionally be helpful for those who do actually signal, especially for lane changes. If a driver in the lane you want to merge into is too far forward to see your tail light signal, they may still be able to see the mirror signal. Most drivers seeing this will either speed up or slow down, not wanting to hang next to someone they know is planning on merging over. Mirror signals could even come in handy for alerting cyclists who like to split lanes or pass on the right at intersections.