I don't know. I guess it's handy when you're in a full tuck and can't easily see the speedo but it's purported advantage (being able to see rpms and speed in the turns) is of little use to me as I ride mostly by feel. The other thing is that it can either (depending on position) block your view of the inside of the turn or merely distract your eye from it.
It's loaded with geek appeal but, for me, not much practical value.