...for understanding who you are designing for.
I've been involved in too many discussions about a design (either after a usability study or a review or during the design process) when we are discussing something that doesn't work or doesn't feel right. Maybe we ran a usability study and observed that nobody could complete a task with the design, or they had a crappy experience. We discuss alternatives to improve the design and sometimes, someone will say 'But we'll get feedback from the blogs that this new design will suck!'.
Are the people who read and respond to your blogs the very same people you are designing for? How would you know? What do you know about who reads and replies to your blog? Contrast that with what you know about who you are designing for. Hopefully a lot more than the user name they use when posting replies to your blog.
That's not to say that feedback you get from any source is not worth paying attention to. On the contrary - it can be very useful to learn about the parts of your product that people are paying attention to or the areas that they feel passionate about. I don't want to suggest that if someone complains about something you shouldn't do anything about it just because you don't know much about the context in which the compaint arises. I know how strong the urge is to deal with negative feedback and how strong the desire is to reach out and try to please everyone that has an opinion of your product.
But your response has to be measured. If you really truly understand your customer, you'll know if the feedback you are getting from a blog posting is consistent with the feedback the person you are designing for would provide. But you can only really achieve such an understanding by interacting with your users, observing them, hanging out at the same places they hang out, doing what they do. You can't expect to understand them from replies you receive to blog posts.