I don't know about 'not the greatest singer'. In Niagara Falls on the second night before YBTTC he said 'I could sing these songs perfectly when I recorded them but I was a lot younger then'. I chose to see that as merely a statement of fact rather than putting himself down. He does praise Don's voice all the time, whereas I can't recall one instance of praise being directed the other way, just as his voice is never praised in books about the band & rarely in reviews.