Even if they don't cure all diseases - if they advance our understanding of some of them, or help cure some, then humanity will be advanced by that much more.
Well, maybe. It's a worthy goal in the sense that eliminating disease would be great. But they may just end up blowing a lot of money that could have been spent more profitably on reasonably attainable goals.
The first emperor of China spent a nontrivial amount of his country's economic output trying to find the secret of immortality. Would that have been a good thing to learn? Certainly. Was it worth trying? No.
1. We don't understand the consequences of this.
2. Our notion of disease has changed (as a previous poster noted, think about our thoughts on homosexuality)
3. Technics used to address disease have historically had some severe unintended consequences (e.g., antimicrobial resistance)
While your example seems fitting at first glance, I don't really think it's relevant. This is private money spent on something that may be a public good, and in this case we do have a basis for saying that, yes, we can eradicate diseases, we've done it before and we're getting close again with others.
All I'm saying is you can end up wasting a lot of money by stretching for a goal that's too far ahead of current technology, and in that sense it might be a more efficient use of money to attempt something more near-term.