Is Sanders an Asset to Clinton?

In a surprising turn of events, Bernie Sanders has kept his campaign alive by beating Hillary Clinton in Wisconsin. On the Republican side, Donald Trump won again and is now the last man standing as Ted Cruz and John Kasich bow out. Is it time for Bernie to bow out, like Cruz and Kasich, so Clinton can focus on Trump? Or does Sanders position in the race as the counter point to Clinton help refine her message and her presence. In that way is Sanders an asset to Clinton?