I write "#1" with confidence, as if I know I'll have additional reasons and this post is the first in a series.
"Irrelevant," because these will not be reasons that ought to have any influence on whether you should vote for Obama or not. (If you want to know what I'm doing and why, that post was yesterday.)
So here's the reason: It will be interesting to see what Obama does with himself if he is suddenly plunged into the circumstance of not being President. If Romney loses, no one will watch him, seeing what he does. He'll melt back into the general population. He's 65 years old. His accomplishments are accomplished. And if he never makes it to the presidency, he'll be a former candidate, not a former President. He doesn't even go back to an existing public office.
When's the last time a major party candidate lost without having a political position to which he could return? Not McCain. Not Kerry. They went back to the Senate. Oh, no! It was Al Gore! He found things to do to interest, entertain, and torment us. He would not be ignored. But he was a young man — young for a presidential candidate — only 52. It's possible that Romney will undertake some grandiose project, but it's nearly impossible to picture him going Gore on us. It's safe to predict that if Romney loses, he'll gracefully concede, wish us all well, and retire into private life, within which he'll be a loving husband, father, grandfather, and great-grandfather, doing manifold good works that will receive no publicity.
But Obama! He's only 51. He was too green to have become President when he did, and he stumbled and lost — if he loses. He had so much promise that he hit the top early, and now all that ability, brought to bear too soon, goes forward into prime years... for what?! That's what's so interesting. We'll get to see what. We only get to see what's down that path if he loses: an irrelevant benefit to Obama's losing the election.