For short 1-2mile W/L they are tricky to use and probably are a waste of time while you race. I think they are good to look at the relative angle changes as wind increases and decreases vs speed.
For distance races they are imperative because your sailing angles are so important. It is pretty easy to tell when you are beating if you are moving but when you have to crack off and sail 57deg for 4 hrs in 17knts the polars give you a good idea of what speed you should be making. This also will tell you if you are over powered (going over your polar) or under powered (not hitting your polar). Either case you want to change your sail combo to match the polars.