Polls show that 85% of Bernie's supporters already planned to support Hillary. So who cares about the remaining 15%?
Care. Care deeply. There won't be party unanimity for any candidate. But the difference for Hillary between the historical average of 15% renegade Democrats and a possible reduction to only 7% renegade may be the election.
Beyond the practical impact, there's a lasting political question about the role and reliability of progressives within the Democratic Party. Occupy Wall Street disappeared. Bernie's profound success has left the Party with a giant, incipient progressive power in its midst. Ignore it at your electoral peril. It's a legacy and an important one. But a sizable defection away from Hillary will strengthen the non-progressive push to ignore the left.
Note that, so far, no mention of the two conventional arguments for joining up with Hillary. First, she's progressive enough by history and by her recent accommodation of the Bern Babies. Second, the Donald is too awful to permit progressives to throw the election his way. I happen to share both arguments.
But the long-term benefits of Bernie's candidacy are really what's at stake. There's a tension between outside forces and mainstream politics. The Civil Rights, feminist, environmental and other movements eventually changed the lives of real people when they forced mainstream Dems to take up their issues. Bernie understands this and, as predicted, is doing what he can to move Hillary's progressive numbers up.
The Sandernistas will either be another one-off, like Occupy, or will be a permanent part of national elections, like Movement Conservatives. So embrace Hillary with joy and enthusiasm, or rationally calculate the stakes, or hold your nose and vote for her. But vote for her you must.