Voting Clinton is not being used by being scared for a Trump. Voting Clinton is settling on the compromise candidate of the left.
Except it is. Rather than voting self-interest and hopefully showing the Democratic Party that "hey, the demographics of the party are further to the left than you think, run a more lefterly [if that's not a word, it should be] candidate next time", they vote for the "safe" pick because the alternative is billed as too hideous to even consider. And SG's right, this ain't the first election that's happened by a longshot. Hell, every electoral cycle is spun as "the fate of America rests on your vote!"
A major purpose of a primary (beyond the obvious electoral one) is to give parties an updated view as to the demographics of their electorate. But that's not how it works, because everyone in the establishment urges people to think about the long game and who's most electable in November. I would argue that if the majority of your party choose someone who isn't electable in November, you need better candidates or you don't deserve to get into power. A strong showing from a Bernie Sanders (as compared to similar past candidates like Kucinich or Dean) should be a message to the party that its membership has shifted further left. But you and I both know there will be little to no movement by the Party to the left, because that goes directly against the people in charge of the money spigot.
If anything, the Republicans seem to be the ones finally acquiescing to the demographic information they got from their primary -- their primaries told them that their membership, by and large, embraces an anti-immigrant rich guy who dogwhistles to white supremacists, and the Party apparatus, as much as they're probably horrified at a personal level, are responding to that (either by falling in line, or abandoning the Party altogether). And if Trump truly isn't electable, then they'll rightly be shut out of the WH. Though I still have this ugly suspicion that neither of them are "electable" in the sense of gaining a majority vote on the basis of their own candidacy, and rather we're seeing a referendum as to who's more unelectable.
I've said it before, if the Dems had run anyone* other than Clinton, this would have been a slam dunk.
And if the Republicans had run...well, maybe not anyone, but a not-horrible person like Jeb Bush or Marco Rubio, this would also have been a slam dunk.
*Anthony Weiner comes to mind as someone who would do as badly or worse at this point.