Advertising and WOM are useful at different stages of the sales funnel.
Relying solely on WOM also gives control of your marketing to the middleman, which can be very expensive (this is common among single-channel campaigns). The platform that distributes your campaign would have too much leverage against you, which is particularly troublesome in WOM, because you cannot craft the message that is being distributed.
Advertising can help both brand evangelists and your regular audience to learn your selling points, enabling WOM.
Remember that, in the case of WOM, you can only provide building blocks that will be assembled by those who will give the advice, using their own language to get your points across. Advertising is good at enabling this, along with preparing WOM recipients to understand and react positively to your brand evangelists.
Your ROI will be higher, in the case of WOM, if both parties are on the same page: otherwise your evangelists would have to overcome negative opinions, which is difficult for someone without formal sales training and lack of proper motivation.
Finally, It’s important to differentiate endorsements, social mentions and testimonials from real word of mouth, in which someone we already know is giving us a tip that may influence our buying decisions. Testimonials and endorsements from strangers that are seen by thousands are not WOM, but ads that attempt to create a better attitude towards a brand.
For WOM to exist, we need to know and trust the source of the information.