Someone using your stuff on their blog and writing an actual article about it is worth more than about 100 credit links to me. I've definitely gotten more traffic this way than some silly link in a footer. And, it's quality traffic because those people are coming for a good reason.

As a theme developer, I too used to hate people removing my credit, but after a while, I got a little smarter. Having 100s or 1,000s of links on unknown blogs doesn't do me much good. Making it easy enough for Jeffro to change his footer text and write an article about how cool of a feature his theme has is worth much, much more.

It's silly for themes to not give users an option to disable this (unless the theme doesn't have an options page). Don't even get me started on plugins.

Oh, and users will just like you a lot more by giving them a choice.