Quite honestly, some of the greatest artist of all time were Bi-Polar. While on the high end of the Bi-Polar swing they write, paint, write music or whatever in great abundance and with great focus. The Bi-polar driven focus often leads to great results. The problem is when they swing the other way the destructive behavior kicks in as they get depressed and moody. When the disease is controlled properly the self destructive side can be stopped and tho the person may not get manic and write or paint as much, they will still be able to do so, perhaps not with as much output.
BTW Morrison's "Poetry" is totally over-rated IMO. His voice? It fits his band, but I don't know that he had great range or that he knew how to use his voice as an instrument the way Robert Plant has. JM was a showman and a performance artist of sorts. A great one at that. Robert Plant is an artist and a tremendously talented singer. Listen to The Strange Sensation album, In Through the Out Door and Presence and even Raising Sands and you can see he knows how to use his voice as an instrument. Morrison, while an intruiging persona, could not hold a candle to him vocally.