Dave always knew a good song when he heard one, and I'll bet he knew that S&S was a winner. Who wouldn't want to be the guy to sing it? It was made for Bob, however, and is a testament to Bob's unbelievable versatility. On the same album, the guy went from an Appalachian murder ballad to a Sinatraeque love/drinking song without missing a beat.
I'd be careful about assuming that Dave's Up and In version is how he would have done it back in 1958. In the 1980s, he recorded a version of "Santy Anno" that was a whole different than the Trio's original version and don't forget he showed up at the reunion rehearsals in 1981 with a 19-chord version of "Tom Dooley."