G'day David,
One problem with commercial links is fairness: do we list those of the producers who have articles on WP, those who we think major in some other manner, all the ones we can find, or all the ones who insert themselves? I don't have the answer to this--I'm asking in the hope of some rational basis for deciding. (I've been trying to maintain the e-book pages and related pages, and there is a continual barrage of links to both companies who are new to the field and not yet notable, and to those who are not really in the same ballpark at all.)
Fairness is not an issue. Everything on Wikipedia is (or should be) intended to increase the quality of our articles, including the external links. The only metric --- the *only* metric --- to worry about is, "Is this article better now that it has this link in it?"
If the answer is yes, then no worries. If the answer is no, get rid of it. If the site owner then says, "That's not fair", then turn the air blue with cussing.
It's that simple.
Cheers,