You have to remember that ambient/viral/guerilla advertising can be created for a VERY low cost, especially compared to TV ads etc, and advertisers don't have to fork out out huge spends in magazines and TV commercial air time...I reckon this form of advertising really does have a better impact on an audience and creates a much bigger buzz.
Take this ad for example. This is a photo-realistic image from high above a city which has been slapped across the floor of a lift. This ad was created by Wirz/BBDO in Switzerland for a Swiss skydiving school. This stunt attracted a wealth of media coverage and is still making it's rounds on the net. Genius.