And It's a Lucky Number or Why Zappos has 13 Blogs by Jeff Bullas

So why have thirteen blogs? A very good question that may have some answers.

1. Because they have the resources and they just “can”
2. I have a sneaking suspicion that they feel they have 13 different persona’s (market sectors) that buy their products  so they have a blog for each type of customer
3. It will make the company look real and transparent
4. It will position them as a “thought leader” in different sectors
5. They realize that interesting and unique content including video will  attract attention

via jeffbullas.com

I agree with the critique that we have talked too much about Zappos...but they do offer some great reasons why you might be limiting yourself if you stuck in "corporate blog" mode.