All I have to say about this is that I have no idea why those bloggers don't disclose when they're blogging about freebies. I do, because there is no such thing as an "unbiased" mind. We all see the world through a prism formed by our life experiences, and it's better to disclose what those are and let the reader make up her own mind about whether or not we're credible.
And yes, I do blog about freebies which suck. Most products, though, come right down the middle and are pretty average, so much so that to write about them would be boring - as would to read about them. That's the only case in which I don't say anything, because I can't be bothered.