The United Nations uses them. Companies use them - but usually those who are unrelated to the product eg a footballer being a brand ambassador for a snack company.
Well, I think the best brand ambassadorships are the ones people like Warren Buffet uses - of course! It's just hardly any companies do this.
You will see the world's richest man talking about products in which he has a vested interest from jewellery to insurance as their de facto brand ambassador.
This model fascinated me. As a leading authority on online trading, the obvious brands to endorse were ones focused on online trading education and customer service. After all, writing 18 books on online trading and a weekly column in the Financial Times, 'Diary of an Internet Trader' makes you care a lot about your audience's experiences and trading education.
And online trading brands agreed, from Goldman Sachs to American Express to MerrillLynch HSBC.