Tactically, It Should Be A Good Thing That Trump Is Our Opponent, But It Saddens Me...

[Bucky]

11. What matters is getting the most votes.

but perhaps I'm being too vague.

My impression is that Clinton is less adept at that in the general election cause she loses too many votes among liberal independents and even within the party base. It's unfair of course, since most of her negatives are a result of bogus Republican smears for the last 30 years. But there you have the reality nonetheless.

The difference, to answer your question: Sanders has more credibility calling out Trump on reform issues and on Trump's flipflopping as he pivots to center (compare his low unfavorables to her 50+% unfavorables). Clinton has flipped enough herself to make such attacks count for much with Trump. Sanders's consistency over the past 3 decades in public service gives him strong bona fides.

Since Sanders is not going to get the nomination, it's critical for Clinton to keep him and his supporters involved and placated. Her smartest move would be to put Warren on the ticket with her. Franken might be a second best choice, but by a long measure. Warren could uniquely re-energize the base and possibly give Clinton some of Bernie's fundraising magic, which she desperately needs.