I think my response would depend on how formal or casual the event was.
I have had a few friends who I invite over and who cancel at the last minute--repeatedly! It's always a migraine or some other health related issue. And I have just come to accept that if I want to keep them as friends, this is what it is. One them I haven't seen in like 3 years!
But, the friendship is there. If she called me in the middle of the night needing something, I would go.
But, if I invite her to a party, the chances of her showing up are about 1 in 10, maybe.
So, if the even requires knowing one way or the other, then I would press for a definite answer. But, if I enjoyed that person's company (and I do enjoy the company of these friends, rare though it be), then I would issue invitations that would be so casual that it would be --well, there's enough if they show up, etc.
I just don't see that it needs to be either/or. They have their lifestyle, or health issues, or whatever. I take them as they come, keep my expectations low and only invite them when I don't really need a firm yes.
That might sound sad, and in some cases, it has meant just letting the friendship go. But, in most cases, it just makes me sad that I don't get to see them as much.