To be honest, DWB aside, Brian's vocals weren't that bad. Hell, on GOK he sounded better than he does these days on it.
I agree. Brian was probably on something, but, he actually appeared "normal". His facial expressions, his piano playing, his voice - it just sounded like he was THERE, as opposed to not all there. Brian seemed lucid for a lot of 1980 and 1981.
I also agree with Aegir that things sound better live, in person. I saw another Beach Boys' concert in 1981 and I thought Brian nailed "God Only Knows" and "Don't Worry, Baby". It almost brought me to tears. However, if I heard a recording of the show, it would probably sound not nearly as good.