I think the defense needing to "prove" their case is just an issue of semantics. Yes, they need to poke holes in the prosecution case, but THEY are the ones who put it out there in opening statements (and no, opening statements are not "evidence") that there is an explanation for everything. Well, even tho they haven't presented the defense side, they really haven't poked holes in the prosecution's case (IMHO), and there is a certain burden on them (the defense) to explain their side that they put out in the opening statement. The jury can't just FORGET the opening statements.