I can honestly say that, on these forums, I have never once looked at a person's post count to judge how valid their advice is, or how much respect they deserve. That thought never even crossed my mind, until it was mentioned here. Also, I don't think I could name any examples of people posting spam to boost their post count.
However I do think that in some ways, it is a bad thing. For example, on a different forum that I frequent, about a completely different subject, I have a higher post count than I should deserve. I am really inexperienced when it comes to said topic, but still post questions and the suchlike reasonably often. This actually does worry me, because I don't want more respect than I deserve, based on my post count, in case somebody listens to my advice over a more experienced person for example.
On the other hand, if somebody with only 1 post recommended a website or product or the suchlike, it is quite possible that they only signed up to the boards to recommend this product, and that it is just self promotion, or promoting a friend. If someone with 1,000 posts recommended something, I would probably be less inclined to feel that they were biased in this way, and that it was merely self promotion.