Should all companies start their own blogs just because? Don't think so! Blogs offer great opportunities for new marketing, or rather, new PR. But as always one must be clear on the objectives before starting to communicate.
Obviously you need to have something to talk about. It could be the life in the office if you have enough entertaining people around, a high profile company may use it to share and clarify strategic choices or you could share some know-how with the world.
The big difference with old school marketing however is that blogs are not fully controlable. In big organisations different people in different departments will spread their vision from their point of view, that's the nature of blogging. And a real blog has the possibility to comment on blog posts turned on. So your respondents will build your image with you.
Solution: Make sure you have your strategy and objectives figured out, and communicate them correctly and consistently, especially internally. In fact, just get your act together as a business and you won't get into trouble.
It's probably a good idea to have the legal department look at the terms & conditions of your blogs ... you never know :-)
Related reading (in Dutch):
Aandacht voor bloggende bedrijven in De Morgen