Your search for a clearly defined boundary of fair use boils down to this question; would you prefer that Congress decide the issue, or would you prefer the courts decide? I would argue that the Courts are better situated, in this context, to figure out what uses out to be protected under fair use. This is so due to the significant implications of rights gained under fair use. If you accept that copyright generally is a good, productive, successful inducement scheme to yield creative works, then taking some of those rights given under copyright away shouldn't be done lightly. The more you expand fair use, the less valuable a given copyright will be to the original author. Given that Congress has to act in a uniform manner and isn't able to feel out the edges of the ramifications of fair use, I posit that courts will produce the most fair and practicable rule for both parties.