I think it depends what we are talking about when we say companies.

A web start-up like WooThemes is a good example of when having 'personal' Twitter accounts helps. We know Adii from his blog and can therefore associate with him. He is arguably a larger brand than WooThemes itself.

On the other hand, would it be the same if the CEO of a large, faceless company had a Twitter page?

On the other, other hand - if Steve Jobs had a Twitter profile...

Typography is the web