ArmedBear - In my opinion, a well-made 16ga is just about the perfect compromise of payload and weight for most upland birds. I tend to find 20ga shotguns (of which I've got a few... new and old) to be almost too light. I have to actually shoot them a little differently, and really concentrate on keeping the barrels moving through the shot. 12ga guns, while extremely versatile and effective, tend to weigh a bit more than a properly scaled 16ga. It's not a bunch, but just enough to make carrying a 16ga just a bit easier.
For example, when I'm grouse hunting, the shooting window is very, very short. You basically have to always be ready to shoot. My 12ga guns tend to be very tiring to carry. The 20ga guns are lighter, but I tend to not shoot them quite as well, especially on crossing birds. The 16ga seems to be "just right".
I agree with you that modern 16ga guns tend to be somewhat lacking. The abomination that Remington came out with a few years ago was essentially a 12ga, except with a smaller hole in the barrel. It seemed like the outside diameter remained the same, so the gun weighed MORE than the 12ga. While it should have felt more nimble, the smaller gauge gun was a pig.
Now, a lightweight 12ga does have a lot going for it. A couple years ago, I came very close to picking up a Guerini Magnus Light 12ga for pheasant hunting. The thing weighed about what I'd expect a 20ga to, or less. Frankly, I think that PJR's 12ga on a 16ga frame sounds great.
One nice thing about a 16ga...
The shells fit tightly in vests with elastic shell loops. Often 20ga shells are too loose and can fall out.
Oh, and don't forget that the "more is better" crowd infects the 20ga and 28ga community as well. The 3" 20ga magnum loads and the 1oz 28ga hunting loads are pretty common... and awful in my opinion. Take a lightweight, sweet shooting, good patterning gun and turn it into a hard kicking, crappy patterning mess. No thanks!
BTW, one of my dream guns is a 16gax16gax30-30 drilling. A stag engraving on the bottom of the action, with grouse on one side and woodcock on the other. 25" barrels with a Greener action. Yeah, that would just about do it.