It's different. Cruz and Rubio aren't mathematically in it, never really had a prayer once the ball got rolling and never scared Trump. Sanders is still mathematically in it (though chances dwindling) and he scares the holy dog shit out of her. Apples and oranges.

As for incentive, there's plenty. If he won the debate, he'd have beaten the guy that scares her, giving him momentum. She'll be on the defensive for ALL of the generals no matter what, but it would have taken him from being on the offensive to being on the offensive with a sledgehammer. If he sensed he was losing, he could have taken that opportunity to try to woo some votes out of it by meeting on common ground with Bernie on any point he possibly could. He could have come away with more votes on his side whether he "won" or "lost" had he played it right. Especially if Bernie doesn't move beyond convention.

Reality is, if Hillary gets the nomination she doesn't really have a path forward against Trump. Nothing can be said about him that will harm him with his voters. Anything she says about him is literally true of her and in many cases far worse is true of her. He's got nothing to hide and doesn't give a fuck what people think, she has a massive number of things to hide and duck/dodge or run from. He's got all the cards in the deck. She'll have lost near enough as doesn't matter to half the democratic votes from the start and virtually all of the independent votes. He could have wooed some away from Stein and cemented his victory (assuming she is the nominee) even further than it already is.

Not to mention it would have taken literally all the media off of her and limited her chances to do damage control with a number of things she's done to screw herself in the last couple of weeks, not to mention what's going on with the criminal investigation. Major missed opportunity for Trump.