When reefed, why would you want your foot (bottom 1/3 of the sail) anything but tight and perfectly flat??
I think it was Stu who said it can be good to add some draft to a reefed main. Not sure if I'm remembering that it was him, though...
Perhaps for sailing through a lull in the action when you want to add power but were not willing to shake out the reef.
I'm just spouting off hypothetically. 
The problem that I see with your windsurfing analogy is that most windsurfers are basically planing hulls. Here we're mostly talking about displacement hulls. Although you may, as mentioned earlier, want to add a little draft to power through the lulls by the time you've reefed your sail you're probably at or near hull speed.If it's beneficial to trim a full mainsail, why wouldn't it be equally beneficial to trim a shortened sail? The wind range for a reefed sail can't be that narrow to warrant flattening the sail as if you were ready to put in a second reef. I look at it this way ... When windsurfing, I'd carry at least four sail sizes on some days just to be able to have the right sail for the conditions, as conditions changed. The trim on all 4 sails was equally important. I didn't focus on setting the largest sail correctly and then just make the other 3 "flat". In fact, the smaller sails were probably more important, trim-wise, as the fun-factor increased with the wind!
I'll have to admit, though, I'm not technical about the trim on my mainsail once I've reefed it.
