One of the most distasteful aspects of legal practice is that most legal arguments take the form of factual statements (or at least claims of fact), but the factual content of the speech is secondary to the effects it's hoped to have (on its listeners).
So put yourself in Nesson's shoes; he's doing this case pro-bono, and therefore has no real worried about being paid for his time. Thus, Nesson has no incentive to artificially shorten the length of his defense (out of concern for working longer than his client can afford).
Nesson also wants to win this case.
So you've got someone for whom: time is no real object, and winning is a goal.
In that circumstance, what's the harm in trying a "fair use" defense? It probably won't fly, but it's one more thing to try, and one more thing for the plaintiff to have to argue against (thereby further draining plaintiff's resources, because plaintiff is paying for their representation's time here).
It's not entirely harmless, sure -- a few too many wild conjectures and Nesson's credibility in court may be somewhat tarnished (though, even there, keep in mind Nesson's (1) something of a legend and (2) already known as a loose cannon, so compared to a run of the mill attorney the impact of one or two wild arguments on his in-courtroom credibility is harder to calculate) -- but it's one more thing that might work and it's one more way to try and exhaust the plaintiff's will to proceed (and I guarantee you this is not the only line of defense Nesson's got).
So, regardless of whether (1) Nesson thinks file-sharing is actually fair use and (2) Nesson thinks he can convince anyone else file-sharing is actually fair use (note: (2) is independent of (1)) it's possible to reverse-engineer the thinking that might make the risk/reward calculation on advancing this line of argument look promising.
But, due to the practical constraints of legal speech, you'll never see any of these statements:
- your honor, I personally don't think file sharing is fair use and I actually don't think I'll be able to convince anyone else in this courtroom that filesharing is fair use, but in the interest of exhausting my opponents with every hare-brained scheme I can conceive I'm going to spend a couple days arguing the point. Let's proceed.
- your honor, I personally think file sharing is fair use and am savvy enough to understand I won't convince anyone here of that via any amount of argumentation, but I am cocksure enough of my argumentative abilities that I won't say anything so immediately laughable that all y'all will be hard-pressed to give a reasoned explanation for why you should shut this line of argumentation down prematurely so you should plan on hearing me out over the next several days.
...or anything similarly candid, ever. Instead, you'll just hear something like what you've heard, which is a straight-faced assertion with no qualifications or caveats; it might be more honest to give a statement like one of the impossible statements above, but giving the statement such a form would be self-defeating in light of the intended effect of making any such statement.