What sort of big decisions does knowing you have Huntington's disease help you make? It's a debilitating disease with no treatment or any way to prevent it.

I mean, all of them beyond the day-to-day?

I plan my life expecting to live to an average age, as I assume most of us do. I save for retirement, I make investments that will pay off at times useful to me.

If I knew I had a serious condition that changes my quality of life and life expectancy, that means I need to change my life plans. Sure, it may not be 100%, but working off the most likely outcome seems sensible.

Not to mention, no treatments right now - if I know I've got something, I can follow developments and if treatment does become available, I can do something. Obviously, you could monitor this if you knew you were higher-risk anyway, without testing, but not everyone knows that.

Knowing about a chronic, untreatable, future illness can help you decide whether or not to have children, and if so, whether to have them earlier than you might otherwise have planned. Likewise it can influence you to accelerate the pace of other life plans (like traveling, or setting up a nonprofit in your neighborhood) and scrap other lower-priority plans.

That... seems pretty obvious to me, does it not to you? I don't know, I for one would knock back plans I might save for retirement to much earlier in my life.

Estate planning implications, savings implications, family goals...

Hopefully at the very least even without testing to go from 50% to sure knowledge, that 50% number is enough to get people to create wills/trusts. Save your survivors a lot of trouble with just a little up-front time and cost.