For people who would vote for Clinton only with the nose held very firmly closed, it's easier to do that if one has had the chance to express one's real preference by voting against her in the primary.
Furthermore, Bernie has energized many new voters. Some of them, if Clinton is the nominee, will go back to seeing only two corporate parties that don't care about them, and won't vote in the fall -- but at least some of them will stay involved enough to vote for Clinton, however unenthusiastically.
(Side note to the admins: The DU spell checker red-flags "unenthusiastically". If Clinton is the nominee, you're gonna have to fix that.)
Even in the states that have already voted, Bernie's supporters are more likely to vote for Clinton in November if they feel that Bernie has been treated fairly. If he's pressured to drop out, it will leave a sour taste in many mouths.
You complain that he's "causing divisiveness." No, he's not causing it, he's capitalizing on it. The divisiveness was always there. Back when I had no idea that Bernie would actually run, I was unhappy with the prospect of Clinton as our nominee, because of her conservatism -- and I think there are millions of other people who felt the same way.
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