All That And ...
Everything everybody else said... and also because, when you're in a airport with three kids and it's 1am, and you've been in transit for 32 hours, and there's a snow storm and you don't stand a chance of getting a cab for at least two hours, and everybody's tired, your kids aren't numb to bribery and so it works. And because in those rare situations when you need to yell, your kids will know that the situation really is dire.
We're very GD, and I believe strongly in all of the principles behind it. It's how we naturally parent, and to a large extent, it's how DH and I were raised. However, in moments of desperation, when we've got no cards left, bribes work because our kids aren't all bribed out. And when the kids are doing something genuinely dangerous, or there is a real emergency, a raised voice actually means something because our kids aren't yelled at all day about every little thing.