A good reason to let people in your company blog is for PR reasons. Putting a human face on your company really improves how people view your company. You show that it's actually made of people with opinions and views, rather than money hungry MBAs. This worked wonders for a company like Microsoft. They were (and to an extent are still) seen as an evil empire, but that has improved a lot over the past years because many Microsofties started blogging. People such as Robert Scoble, who even wrote a book about it, is a good example of this. This idea is not new however. Today I read something interesting in a book I'm reading for my studies: An Illustrated History of Britain (by David MacDowall) about Queen Victoria of England (page 144) who was a very unpopular queen, until 1868:
One important step back to popularity was the publication in 1868 of the queen's book Our life in the Highlands. The book was the queen's own diary, with drawings, of her life with Prince Albert at Balmoral, her castle in the Scottish Highlands. It delighted the public, in particular the growing middle class. They had never before known anything of the private life of the monarch, and they enjoyed being able to share it. She referred to the Price Consort simply as "Albert", to the Prince of Wales as "Bertie", and to the Princess Royal as "Vicky". The queen also wrote about her servants as if they were members of her family.
Of course there was no Internet back then. Electricity was barely invented. But still, she wrote openly about her issues and the people she dealt with. It gave insight into her life. And it worked. She became very popular.
So Queen Victoria may be one of the front runners of what today we know as corporate blogging, or rather: royal blogging.