In some situations it is an advantage--I used to ride DH mountain bikes, and I had a bike with a chris king headset on it that lasted through 6 seasons of hard riding, and is still going strong. Although another headset might have lasted (though I've had ones that haven't), it's not really worth risking destroying a $3,000+ frame by ovalizing the head tube. I think it makes a lot less difference on bikes that don't take quite so much abuse.