Hanging a light on potential bias builds trust with the reader. He may have linked to the best parts, but how do I know that he didn't pick the parts that make him the most in referrals? You have to trust the author or usefulness of the information is compromised.

BTW, I'm not saying the author is taking advantage of his readers nor am I saying he has an obligation to disclose his referral links. I'm just pointing out the difference that it would make and suggesting that all bloggers consider it.