Why should Bernie drop out?

I understand why Democrats are urging Bernie to shift his attacks to Trump, but I don't understand why some are urging him to drop out of the race. Okay, I can see why some Hillary supporters are nervous. In football, a team can lead by 3 touchdowns in the third quarter and every now and then the other team comes back in the fourth quarter to win the game. Bernie's chances of winning are slim, but not impossible. Let's face it -- there are very few primary voters who actively dislike him. They may feel he can't beat the Republicans or believe he hasn't really played much of a role in their lifetimes, but it's hard to find fault with him. So, he remains a viable candidate. But, that means the "worst" that happens is that he gets the nomination. Is that really so bad, Hillary supporters?

Yes, there are obnoxious Bernie supporters who compare Hillary to Nixon, Reagan, and Bush, but those people exist no matter what. You have Hillary supporters who gloat and tease Bernie supporters when their candidate loses. You have the angry mobs of sports fans who are playing politics, who want to rub the losers face in the mud or demean the winner in anyway possible. But, the reality is that the people engaged in that kind of meanness have their own shit to deal with. We don't quite know what drives them. We don't know what would drive by some accounts 11% of Hillary supporters and 19% of Bernie supporters to oppose the nominee if their candidate doesn't win. But we do know that in 2008, there were similar feelings after Obama won the nomination, and in the end only about 10% of Hillary supporters voted for McCain. The nominee will lose some support, but not all that much in the end.

But, the point is the nominee stands to gain so much more. Keeping Bernie supporters engaged in the race reduces the risk of apathy and disinterest, allows them to be part of the changing political landscape, and gives them a chance to be heard. If people are heard, they are more likely to listen. If all the states get to fully participate in the race between Hillary and Bernie, then even those on the losing end will be more open to hearing what their candidate and the nominee have to say. If Sanders stays in the race regardless of his odds, then when he and Hillary come together and urge unity, there will be far less apathy, and the votes gained will far outweigh those lost. Not only that, if Sanders supporters stay engaged, they may find ways to keep their political revolution going after the election, which can only benefit the Democrats.

Souces:

http://www.cbsnews.com/news/hillary-clinton-leads-bernie-sanders-in-national-cbsnyt-poll/
https://en.wikipedia.org/wiki/Democratic_and_liberal_support_for_John_McCain_in_2008