Here's a rare picture: a 2009 limit of Iowa roosters (they're hard to come by this year), and, rarer yet, me with a 20 gauge. I can't remember the last time I shot a rooster with a 20 before this season.
There are so many light 12 gauges now that weigh the same as 20s that the 20 gauge might just be obsolete. My Benelli Montefeltro weighs 6 ¾ pounds with a 28 inch barrel; alloy framed O/Us like the Browning Citori Feathers weigh even less, as does the wonderful Benelli UltraLight. The near-mythical Ruger Gold Label side by side - I am one of the few who have one - weighs 6 ½ pounds in 12 gauge.
I can shoot reloads as light as ¾ of an ounce in my 12s and load them up to 1 ¼ for pheasants. For all practical reasons, there is not much a 20 gauge can do in the uplands that a 12 can't do better. Add non-toxic shot to the equation and the 12 gains an even bigger edge. The exception (as is the case in this picture) is if you load up the 20 with HEVI Shot, but that is an expensive way to go.
I like 20s for dove hunting when I get to go, and I have a 20 gauge 1100 with a youth stock on it that I think is the ideal gun for new shooters on the clays range.
All that said, of course, being a gun nut isn't about logic. I like this gun - a Caesar Guerini Woodlander -- and will keep hunting with it, even if it means leaving "better" 12 gauges at home sometimes.