We probably shouldn't put the main focus on rare disease but on genetic manipulation in general. It would be much more valuable to be able to cure arbitrary genetic defects than just the ones that are somewhere between uncommon and extant.
I mean, this has definitely happened in the past; study of various rare genetic diseases has elucidated and illuminated the processes by which diseases occur at a molecular level, and often have utility far outside the original rare disease.
As to whether that means it's an effective payoff if you put more money in rare diseases (and therefore less into non-rare diseases), I can't say, so instead, I have a portfolio with a small but not tiny amount for the collection of all rare diseases.