I don't use power in a race. I know others do, particularly to moderate efforts in breaks or TTs. For me, knowing the numbers is more detrimental than helpful - it's a mental game. I tape over my computer in RRs and crits. For the track, computers are not allowed to be visible, so mine is under my saddle.
The value comes after the race. My coach can analyze my numbers to see how I did in a real race setting.