In my experience that hasn't been true. Had the 18-135mm and the 18-200mm and, in my personal experience, have to say that the 18-200mm gives much better pictures.
It has been pretty visited that the 18-135 is sharper than the 18-200 at the corners at various focal length. Check out Thom Hogan's review. Here is a little side by side comparison of the two i did some time back. ( edge comparison of corner of a pic)
But. Of course IQ was not i bought the 18-200mm for. It was for the convenience. And yes it can definitely take keepers.
Ryan