I am sorry about your wife, but I am glad that she is mentally sharp.
However, some fraction of Parkinson's disease sufferers, as the get older, do develop dementia, and of course some don't, but it appears that as the disease progresses, more and more do.
http://www.parkinson.org/pddement.htm "Dementia, a global decline in intellect, is among the most feared complications of Parkinson disease. The behavioral consequences of dementia can be painfully obvious. Patients may be confused, disoriented, unable to be left alone. They may be agitated, delusional, moody, and disinhibited. They usually can't sleep at night, and can't stay awake during the day."
You are correct that patients with degeneration of the substantia nigra also may have a degeneration of other areas of the brain. (depending on the cause of the degeneration)
However, the clasical parkinson's refers to the brain's ability to produce dopimine, which has nothing to do with these other symptoms. (Other than whatever caused the degeneration in one area, may very well effect another)
Some old folks, regardless of the death of dopimine producing cells, have degeneration of areas of the brain that produce dementia. For the most part, this is just old age...
It is an effect that relates to ageing and the overall health of the brain, not specificly parkinson's disease. It just happens to effect some of the more sucseptible people. However, it's not parkinson's disease. The lack of dopimine (which is what Parkinson's is.) has nothing to do with dementia. It just can occur in some of the same people and may be triggered by the the same underlying problem.