Sometimes you will find comments on your blog which link back to commercial sites and are there to promote a product or boost the google-juice of the linked to site. Are these spam? Should you remove them all?
Here are two comments which were recently posted on Fractals of Change. IMHO, one is legit and one's not:
"what a nice post Good luck!!!!!!!!!!!!***[commercial name removed] Challenge"
"We have recently created a free link exchange directory http://www.link-exchange-submit.com. Providing people use it fairly, it should help you with your SEO link building. It is automatic in terms of your link and suggested category being added. But it is also moderated daily in terms of spam removal."
The first comment is clearly spam in that it contains no internal reference to the content of the post. For some reason, the people or bots that post these almost always stick in a generic compliment thinking, perhaps, that you are starved for praise and will leave the comment posted. This one was deleted and reported to Typepad.
Since the post that the second comment was attached to WAS about links, the comment is right on target (although it could have been posted by a smart bot). My post was against link exchanges and this is clearly a contrary view; but that's fine – good, in fact. The service which is being promoted is something a blogger thinking about link exchanges would want to look at. So the comment is likely to be useful to readers of the post. It gets to stay.
Similarly, a post of how to make cheaper cellphone calls abroad is festooned with comments containing links to SIM providers and cheap or free calling services. Again, anyone interested in the post is likely to be interested in the links. Sure, they serve a commercial purpose and I don't get paid for them as would get paid for ads. But the comments expand on the post and make it more useful to readers. On FOC, that's the spam/no spam criterion.