You make a good point.
Not being a lawyer, I've never thought much about strategies aimed at jury nulification (beyond that great, old, anti-PC Jack Lemmon film, How to Murder Your Wife). I can see where it might be very useful to give the jury something to at least pretend to believe.
I'm not a lawyer either, but I watch an unhealthy amount of Law & Order, and I watched the OJ Simpson trial. The defense doesn't have to prove its case, just to create a reasonable doubt in the mind of one or more jurors. I've seen enough human behavior to know how people respond to a half-baked theory that they want to believe about a sympathetic individual.