Actually, Joe Biden looked pretty good. In fact, Joe's popularity has gone from 1% to 2% last week to 3% today. At this rate, he could win the nomination by the year 2032.
I do think sometimes there's danger in guest appearance mania. I've seen too many examples that sound cool on paper, like 'Oh, get that guy to sing the hook on that guy's song,' and then that's all it is. It's a cool idea that sounds good on paper.