My answer would be yes. For now I am stuck with closed source applications like MindManager to create mind maps. I have tried a few applications for writers like Writer's Cafe. They are useful, especially in keep track of bits and bites lying around.
Mindmapping with pen and paper is possible, of course, but it does have one major drawback. When you want to change something you have to draw your mindmap again.
My advice for an open source version of mindmapping (mind you, there is already freemind):
(1) make it multiplatform, meaning: it supports at least Windows and Mac and PalmOS, Windows mobile (I carry an old Windows CE device around everywhere, beats the notebook any time)
(2) add extensive text editor functionality to it, so that you can write chapters and paragraphs from within the mind map application (something that you can do with MindManager already)
(3) add a database function (for the bits and pieces)
The building blocks are already there. You have Freemind (mindmap), Abiword (great multiplatform text editor) and MySQL (open source database, also in a lite version if I am correct). The text editor might have some predifined templates tailored to writer's needs.
In my opinion software is a tool, just like pen and ink. The advantage of software is that you don't have to re-write everything when you want to change something. Other than that, it is the user's versatility that determines the quality of use.