I disagree my GT4 has one, its not perfect. However it tells me when I am runnning really rich (on boost.) Goes to green.
I recommend that you install one if you have the time and the money to.
They do flicker about though and will only give you an accurate reading on WOT (wide open throttle) or when crusing at a steady speed.
When they flicker about that means; The ECU has initiated maximum fuel efficiency and cannot establish a perfect air/fuel ratio so it will flicker back and forth. A problem can be deteceted if running rich when cruising. (should flicker)