They may be necessary to get that last few percentage points of efficiency. But I've been riding on flat, double sided pedals for a couple of years now and have no plans to go back. I don't seem to have any more, or any less, trouble keeping up with my usual riding buddies, than I did with SPDs. And, my knees and feet feel better at the end of the ride.
Mark Stonich;
Minneapolis Minnesota
http://mnhpva.org
http://bikesmithdesign.com