I should preface this with the statement that I do understand that HTML Framesets are on their way out, and that they have all sorts of SEO/accessibility concerns.
That being said, I've been considering building a web-based API app, and I really like the format of the Java and AS3 APIs. They both use framesets in what I consider to be a reasonable manner, and are the most readable API's that I know of. The AS3 CS4 API no longer uses frames, but is verrrryyyyyy slooowwwwww.
My other consideration was to make an AJAX-driven (jQuery) site so that I don't have to reload the entire API class/package list on each page load. It would likely reduce the data-transfer to be less than the frameset version.
Am I being foolish to think that this could be a useful application of framesets?