Word-of-mouth marketing works because it comes from a trusted source. Whether stimulated by relevant product experience (iPod), a highly creative ad campaign (Subservient Chicken), or even a degree of mystery ("The Blair Witch Project"), it works because the messenger, the consumer, is more credible and trusted than the advertiser.
This isn't fluffy conjecture. A 2004 Forrester/Intelliseek study found "recommendations from others" and even "consumer opinions posted online" significantly outranked TV, radio, print, and online advertising on the trust scale. This is a big deal because trust and persuasion share a symbiotic relationship. Research from Jupiter Research, NOP World, and Capgemini echoes this theme.
If anything, the importance of trusted recommendations is on an upswing because advertisers continue to alienate consumers with intrusive, interruptive, even invasive ad formats. Consumers are clicking away and reaching for ad filters. They're increasingly cynical about marketer duplicity.