Michael Feathers is doing to Fit something like I'm hoping to do to Rails in this interesting post. Ward Cunningham himself weighs in with some of his profound insight in the comments:
Authors of large programs are wise to modularize things like application sequence, data access and input/output conversion. Where these modules exist in an application, it makes a lot of sense for Fit Fixtures to use them (after first testing them, of course). Be careful not to make your variation of Fit a platform for Fit-only modules that discourages modularity where it matters most, in the delivered application.
Thick and meaty stuff that goes right to the core of the issue, both for Fit and Rails, I think. I'll be thinking about that advice, and indeed the whole post, for a while. This is wonderful stuff.