Despite the vituperative trend in this thread, I'll respond to Gord's question instead of badmouthing those dirty scumbag racers !
First, IMHO (actually, I suspect that neither I nor most other users of that expression are really very humble), the best lessons are learned not crewing
on someone else's boat, but by taking your very own CRUISING boat out and entering some club races. This way, you are in control of the dreaded shouting, you make the decisions and then observe their results, you learn to use all those adjustable thingumbobs on your decks and masts, and very likely, you will become a more competent sailor. You might learn that being over canvased is not fast, that big heeling angles are not fast, and that driving the boat so hard that gear
breaks is not fast in the long run... all of these lessons are directly aplicable to cruising.
The need to hit the starting line even in somewhat inclement weather
may give you some good experience, too, and the need to somehow get the boat to actually sail to windward (something a great many cruisers usually do under power) may teach you a skill that saves your butt on a lee shore with a dead engine
some stormy cruising night.
On the other end of the spectrum, raciing may teach you to sail your boat in very light airs, 'cause you can't just turn on the donk whenever the speed drops below 4 knots (or whatever). If you learn that skill, and equip your cruising boat with the necessary gear
to practice it, you might be able to opt out of the "motor boats with masts" gang that we see each time the wind
drops below about 15 knots.
I might add that none of these activities will necessarily lead to winning races, even at the club level, but they will likely lead to being a better cruising sailor in your very own cruising boat.
And finally, none of them will necessarily make you into an insensitive, shouting, beer-swilling jerk, either!
Jim and Ann s/v Insatiable II lying Gladstone Qld Oz