I own a 18-200 + D80. It's real convenient travelling with a single lens and dont have carry a tripod. I had a picture taken 1/2 sec handheld (18mm) perfectly usable. I agree its not a sharp lens compared to those shorter standard zooms. I did own KM 17-35mm , a real sharp lens, beautiful colors and contrast. But it's not fair comparison.
Carrying a bag full of lenses + tripod, and chanding lenses can be real hassle.
I may buy a tamron 17-50mm F2.8, but I will still keep my 18-200mm ,