It was a mistake for Trump to accept this imo. The potential upside is minimal and the potential downside is substantial its not worth the risk.
You don't think that dominating the news cycle for the next couple of weeks (and thereby putting Hillary on mute) is a big potential upside?
Any slip up will be pounced on by the media, Dem's, and GOP establishment losers who have sour grapes.
They're going to "pounce" on him no matter what he says or does anyways, might as well go at it with ole Bernie since Bernie's proposals are so full of holes that they make Swiss cheese jealous.