do you think that not relevant content weblink will decrease your Page Rank?? I see many forums with high PR get lots of links in the signature that benefit to the members but not for the forums. However, the PR of these forums are not decreased. from my experience, my website PR 4 and I exchanged link 3-4 not related website with lower my PR, but my PR was not changed (for 2 times google updated up to now, I didn't do anything to improve my PR at that time). Dont say that they want to get visitors from related websites. Because of low CTR on link, I don't think that is good way to get more visitors. Also, people who are loyalty to the website they are in, they dont want to look at other website. For those people who are new visitors, they wont even mind to see the link on these websites. so why many people dont want to exchange link with not related websites ??
Quality and relevance both form the pillars for a strategy like link exchange. If it were about one way links, even non related sites would have been OK. But the exposure increases significantly in case of link exchanges. One more thing to have a look at would be the kind of sites that you are exchanging links with. If these are in the "bad neighborhood", Google might eventually take action against you by lowering your toolbar PR. Ashish