My one vote of course won't tip the balance, but the millions of moderate and independent voters will. Why does he need our votes? Because there aren't enough ultra-progressive types out there to elect him, it's as simple as that. You are NOT the majority of American voters (again, it is part of the arrogance of youth to suppose that you are). When you see polls showing Sanders beating Trump, you are ALREADY seeing the moderates factored in, because if the polls singled out just ultra-progressive types, you'd probably be seeing him with at most 10% of the voters.
So, beware of who you alienate with your contempt. Piss off enough people, and you may find yourself isolated, and on the losing side.
To beat Trump, Sanders needs a coalition. He can do it, but it won't be easy, and it won't be done just by his own supporters. People like me who aren't crazy about him but still feel that he is better than Trump, will ALSO be needed.
No, I won't sit out the election. I'm very anti-Trump and will do my best to beat him. I hope to multiply my one vote by convincing other moderates who might be over the fence to vote for the Dem nominee, and will try to multiply it more by donating to the campaign, and by volunteering my time, for example, going on registration drives to put back on the voter rolls, people who were wrongly expunged from it, in the GOP's concerted effort to suppress the vote of certain demographics.