Both of my grandmothers had Alzheimer's, and both clearly in retrospect tried to hide their problems as long as possible. Knowing might have made some difference in type of care, but not much, and at least one of them clearly explicitly hid it because she was as scared of the potential consequences of e.g. ending up in a nursing home as she was of the condition itself. I'm terrified of getting it, given what I've seen of it, and the prevalence of it in my family, but I'm not at all confident I'd want to know because of how it might affect my enjoyment of the time I'd have left.

At the same time, by the time it becomes a question, odds are the patient may be showing symptoms where it isn't a question if anything is wrong, but whether or not what they have is treatable (that we know something was seriously wrong was the case for both my grandmothers). Perhaps being able to rule Alzheimer's out will be helpful in that respect.