I also used to check my links periodically (because I use manual link adding), and used to see that a lot of my partners removed my site, or the page is not available, or just I cannot reach it from the home page anymore.
I used to write a letter to them and wait for 1-2 weeks until I completely remove their links if I got no answer…
At this times, I rarely exchange links, fortunately my site is referred on other sites (without asking for link exchange). Or I exchange links only with CAP members who are actively use the forum.
It is always sad to see that you are honest and take care of other’s links and they just remove yours after some time (or use disgusting techniques like nofollow) – that’s why I don’t use automatic exchanges.