OK, the comments about riser bars being for the masses and not for serious riders isn't supported by what I saw today in the bookstore. Going through a number of mtn bike magazines, looking at what racers are riding, it's very clear that riser bars are almost the norm, regardless of the type of racing. So there must be a performance and/or comfort justification for the move to riser bars from straight bars, but I just don't see it yet.
Why did racers make the switch to riser bars? Simply to sit more upright?