When i started at my current company their policy was to add site wide links in the footer of every site we marketed/designed. The only problem was that they used the same link text on every page of every site and they all linked back to our home page.
This did help build a decent
PR on the home page but now we are absolutely nowhere to be seen for the link text used, and i mean nowhere! I reviewed this practice about 12 months ago and told everyone not to do it as I'm sure it has led to our poor site performance. We now add one link on the home page, if the client gives us permission, and we make sure to change the keyword regularly. I don't see any problem with this as it brings us business (admittedly not much, but its better than a slap in the face) and its free advertising.
I always thought that if you had a link on an irrelevant site that it didn't do you any harm, it just didn't do you much good. Now i am thinking that if you have thousands of irrelevant links, especially site wide, that this is BAAAD! Who agrees?