On last sat i did races (small mono's) and the skipper had a very nice compass mounted. We (he) started to work out the wind angles/mark settings before the race...
we never once used the compass during the 2 heats we ran (1st and 2nd) before we broke the boat! I think it would have been a distraction, but if we couldnt see the marks i bet it would have been great!
ever since we had a fog fest in Dunedin during the Hangover Regatta 2 years ago (and we had about 15 boats without the ability to see, or even really guess where land was..) i carry a small $2.00 compass with me..
Your bearing always changes to the mark as you go up the course...so knowing it at the start really only helps you identify where it will be within a narrower field. If the RC moves A-mark during the race, and holds up a new bearing to A-mark, it allows you to know for sure if it moved to the left or to the right. It really doesn't help you find it when you're half way up the beat.