A lot of people still perform their duties this way. However, you'll find that most of the faster teams have the crew working the mainsheet upwind and the kite downwind. It's practically impossible to keep from wiggling the helm while yanking on the mainsheet at the same time.

Having a crew run the mainsheet is an instant go fast idea. I usually have the main in my posession when I sail with Eileen as she is not comfortable enough to know how to play it. We are trying it little by little on practice days but...

I went out a couple of times with a crew that knew how to play the mainsheet and the speed increase is significant.

Downwind the spinnaker is the concern for the sheet tension for the crew. The newest cuts require more tension. We run a double ratchet system so she can hold it easily after the sheeting is done. We turn both on in winds over 15 knots. The best part about the spinnaker cats is when the chute tension is that hard your crew is usually out on the wire and can use her (his) legs to do the pulling.

My girlfriend/crew weighs in at 125lbs and will sail with me in all conditions on our Formula 18.

Later,
Dan

For me this is a no brainer. Crew on main upwind. I usually sail single handed. Recently when I had the luxury of a nationals sailor to come out with me I crewed so I could check his style. Out on the trap with the main is awesome. No tiller to thing about and just pushing and pacing to power and using weight to best advantage.
I am absolutely clear that I would love it the other way round also. Just helming with a great automatic main trim.
This has to be fast so long a both people do their job well.
This also has the added advantage that the crew is going to be responsible for any harm he or she comes to and the main can be used to not 'fall" forward.
I bet that the control freaks have a great deal of trouble handing over the sheet thoug. I would suggest they would have more fun calling the shots from the foredeck.