It's not too often I disagree with Webnauts. But this time is one of them.
I design for primarily small businesses and non-profits. Maybe if I was working on huge sites for corporations, with their correspondingly huge price tags I wouldn't need or want to do this, but I see no issue with this as long as the client doesn't care. And not one of my clients has ever said he or she doesn't want the link there. And those links do bring me business, so even if they hurt a bit on
PR I'd still use them. They're directly relevant to what I do, and yes, they're just like an artist's signature.
I don't think they hurt at all. Yes, it would be better to have links from other web design sites but hey, I don't know a single competitor who wants to link to me. Do you?
I think those footer links are probably not as strong as other links but I do think they have value in the scheme of things. Re homepage only, I doubt it matters, but that's just a gut reaction and I have nothing to back it up with. The one way it might matter is if you only have a link on the homepage and the person who likes the site comes in through another page and doesn't find your link in the footer or sidebar, and you've just lost an opportunity to snag a qualified visitor.