There's a story in Politico about Bernie Sanders eschewing traditional forms of debate preparation. He says instead he'll use policy issues, and draw contrasts and not use canned lines.
While all that's good, he should not forgo traditional debate preparation such as having stand-ins for Clinton and O'Malley because he needs to know what they'll say against him. It's not bad to have a prepared line of response because it allows you not to have a moment of hesitation or a moment where you look unprepared for that line of attack from the other candidate.
He also has to know how he comes across on camera to the general audience watching at home. We all remember the debate with President Obama and Mitt Romney, where President Obama hadn't done his debate prep, and it showed. He dropped in the polls after that disastrous debate. He had to do his debate prep to do well in the next debate. That was near political malpractice.
And it would be political malpractice for Bernie Sanders not to do the traditional debate prep. I know not doing it is a part of his shtick as the unconventional candidate, but he'll be appearing in front of millions of Americans, and they need to see he can respond well and handle any questions, attacks, or responses.