Personally, I rely on my black humour from my previous life as a cop.
I watched my mother slowly fade due to alzeimers and dementia prior to her demise all alone (as was her wish) and, believe you me, it was not a pleasant experience.
I see both the humour and intent of the initial remark. Lets not get our collective knickers in a twist over personal interpretation/experiences and their misenterpretation. Why not let our memories of the happier times supercede the sad? That works for me anyways.... 
I agree entirely. No-one would want to have the experience of seeing a loved one suffering from such a disability and I welcome the government's initiative to give extra funding to research into dementia. However, I note that this morning's news report this morning just HAD to put a figure to the "cost" to the NHS of dealing with this problem - thus reducing the measure to one of economics rather than compassion.

Nevertheless, we need to see that humour is a great release and disabilities do lend themselves to black humour. We can cheerfully make jokes on here about Heather Mills and her wooden leg, yet not about dementia. There is a inconsistency, IMHO. Practically any misfortune can be fertile ground for humour, yet, when the chips are down, the vast majority would, at a personal level, feel pain. But we're not talking about individual personal circumstances, are we?
The ironic thing about dementia (having had a close friend deal with a mother who had the condition), is that the sufferer often lives peacefully and relatively happily in a world of their own, whilst it is actually the relatives/carers and loved ones who experience the real pain with the protracted sense of loss of a loved one.
Let's lighten up and see humour for what it is - a dependable crutch in an often painful world.