IMHO IDEs tend to be optimised around a specific platform or language or OS: Eclipse JDT is great for Java, Visual Studio is C++/.NET-centric, etc. They help productivity a lot (again IMHO) if you're only working on that platform, but if you change platforms you have to basically learn a new IDE (or at least a new set of plugins, views, perspectives and I don't know what else for Eclipse).
The advantage of knowing emacs, or Textmate, or vim (my personal preference), or any generic editor, is that the skills you acquire in that editor apply regardless of what platform you're writing for. They're optimised for editing text, and once you master them, you can edit text very efficiently in any language.
There's also Yegge's assertion that great programmers adapt their tools to their working style rather than vice versa. I think this is a win for generic editors, because you customise one editor, rather than having to work out how to adapt four different IDEs to all behave the way you want.