I think this is right. If not, all you'd have to do to sabotage your competitors is create a paid directory site and fill it with their links!
Matt Cutts (google spam fighter) has said that buying links in directories is fine IF: - if the payment is for the review. - if the directory has high standards. Having a paid directory is fine, and so is buying links in a directory. But because directories are usually unable to attract natural inbound links from authority sites the links are not worth a huge amount.
If that was true, then it would be a real easy way of getting rid your your competitors... jsut feed their links to the paid links sites, and they'll get penalised... I'm not doubting what you're saying, but there may be other factors you might not have considered...
Links in footers, sitewide links, links in free directories and any link which a human Google lurker may consider as non-natural may become worthless. Google does not pay attention on what is paid/not paid, but what spoils their SERPs. But, links in directories which demand $$ for a review is just the same as paid links, so it seems funny why Google consider this "review" joke seriously. Maybe because Google ment that only Yahoo and few other quality directories qualify as "review directories". I doubt you can open a new directory, write "reviews for $$" somewhere and you'll be accepted by Google as good directory.
What makes you say that? Just doing link:www.mysite???.com will tell you that it sees at least some of them.