Jon,
This is a GREAT question, and I think Mark is on the right track with his answer. Like he said, most bowl-shaped natural lakes aren't going to have exposed rocky bottoms because there is no depth contour that causes the silt and muck to slip off into deeper water and uncover the hard bottom. You're better off looking for a Volkswagon on the bottom of most of those lakes, as their smooth bottoms will almost always be silty.
Another interesting thing about depth in the winter, and this has always been a mystery to me, something I would love to study if I ever had the time or money. I think I've brought it up on the website before, too. In Western Washington during the winter, pretty much EVERY SINGLE lake is going to have a uniform temperature from top to bottom. The exceptions would be if the lake was ice-covered or you look at the top 2 or three feet that are affected by the sun. This includes big lakes like Lake Washington. So, if that is the case, why do we catch so many more fish deep? I think it's for a few reasons, mainly that we "think" all the fish are deeper because that is what we are told and also because it is probably where most of the food is during the winter. There isn't any real production in shallow water from zooplankton, insects, etc. so no reason for smaller fish to be there and risk bird predation or other dangers.
I've had limited luck when I tried to fish really shallow in the winter on Lake Washington. In February, I've caught a few smallmouth in less than 10 feet of water, and they were every bit as active as the fish in deeper water. But, people still catch a lot more fish deep this time of year. I would love to tag a few fish some day and follow them around in the winter to see how much they move around. I think it is a LOT more than we suspect, especially for smallmouth.
And, oh yeah, I think structure is much more important.
Chris