I think Marty's need is much simpler than that, and more explicitly stated. Marty needs to learn to take risks, and to not let a fear of rejection rule his life.

He says in his conversation with Jennifer in the town square that he's not sure he could take that kind of rejection (if the record company hate his tape). George McFly says exactly the same thing about asking Lorraine out - that he's not sure he could take the rejection.

When he throws his tape away without sending it, Marty is adopting his father's fear of rejection as his own. In curing his father of his fear of rejection (pretty much all of Act II), he cures his own. I seem to remember that there's a line about how one turns into one's parents, and I think that is the key.

We know that Marty's dreams will never some true if he doesn't send his tape. Marty's forced to confront the same fear in someone else, and in talking them through it solves his own problem.

Thanks for this awesome analysis, Scott. I'm knocked out by the elegant relationship between the external and internal stories of these two films. Perhaps that's part of the reason they're both classics.

Nathaniel: I agree with what you're saying up to a point, but feel that rather than an either/or situation, this is a separate subplot, and the one in which the conflict is the most clearly internal...aka the "Character vs. Themselves" part of the story, to the "Character vs. Character" of Marty and George, and the "Character vs. Environment" of his need to get home.