1. Word of Mouth is shown to be more effective than traditional marketing, per dollar spent, and the message is stronger if it is coming from someone you trust.
2. Word of Mouth Marketing is inclusive of other kinds of marketing. Everything you do should be made buzzworthy to turn up the volume on your marketing.
3. Word of Mouth Marketing is not seen as an intrusion (unless poorly done) by the recipient of the message. In fact, people like useful, fun information that they can spread the word on.
4. Even facilitated Word of Mouth will not go far if the quality of the product and service do not back up the hype. If your word of mouth continues to spread, you are doing something right.
5. Word of Mouth Marketing has a lower cost to produce. It is more a philosophy to make everything remarkable, than it is a series of campaign initiatives.
I am sure there are some other benefits, and please feel free to add yours to the comments.