I often ask my clients if they would be prepared to introduce the reciprocating site as their business partner in a meeting? once they look down the list and see they have gambling and porn as their business partners, they soon get the point.
Is the link relevant? Do they actually validate your service/product in the real world? Would they link to you regardless if the link wasn't reciprocal? If you had to make a real effort to gain that link, would you have bothered?