Simple but says it article from Website Magazine about how not all link requests are created equal.
"It’s easy to dismiss many link requests as spam, and rightly so. Unfortunately many less-than honorable website owners will send huge email blasts in the hopes of gaining a few links, even if they are completely unrelated to the website. But many times a closer look through your inbox is warranted. There are plenty of professionals out there making a concerted effort to gain and trade valuable links — and you should too."
Read it here