1 - every post has to be vetted by legal or PR. It won't ring true.
2- you're not prepared to allow any negative comments. Blogs are not monologues.
3- the purpose of your blog is message control -- which is, and always has been an illusion.
More accurately it should be said that a lot of companies aren't set up internally to blog, whether it's because of the interference of politics, legal, brand managers, etc. Of course it's scary as hell to tell the 35 year Madison Avenue vet-turned CMO to start blogging and accept criticism - but part of our job is to also explain the benefits of having a blog, whether it's public facing or internal. There's also a thing called scope, whereby you can define the topics covered of your blog. It's not a free for all message board, for cryin' out loud.
To say they shouldn't is a completely different issue... Very few companies can afford to not open up and have a dialog with their stakeholders.