Because doing it with and not instead of is too hard
Arguements using circular logic can be brutal. Rarely correct, but oh so very effective. However logical Fallacies rarely make good business cases. Sometimes, as Mitch Joel likes to point out, it isn’t a zero sum game and social media and other forms of digital marketing can actually happen as a part of an integrated strategy. However, you get more clicks by declaring the death of SEO and the irrelevance of email.