ADDED: My son John Althouse Cohen emails:
You wrote about how everyone watching the convention is imagining how the speeches will seem to someone else, even though it might be that none of those "someone elses" are actually watching the speeches. The same thing happened when Kerry won the primaries. Everyone was voting for him because they thought he would appeal to someone else. And those voters believed at the time that that was the politically savvy thing to do. But it was actually politically disastrous: if everyone was just voting for him because they thought someone else would like him, then NO ONE ACTUALLY LIKED HIM.
One problem is that if you're trying to choose the most "electable" person, I would imagine that you'd be likely to do it by process of elimination -- by ruling out all the candidates with obvious political liabilities. I think this is the number-one reason why Kerry won the primaries: he was the only candidate who didn't seem to have anything particularly wrong with him. Edwards was too inexperienced; Clark was a poor campaigner; Dean seemed kind of insane; Gephardt was too liberal; Lieberman was too conservative. So they choose the one candidate who has no qualities that would really make anyone hate him. The problem is that he also has no qualities that would really make anyone like him either.