When I used to hear him say those words, I was very young to argue and when I could be was no more. But I asked my uncle the same. He replied that a dimwit most probably won't require more than his necessities, so again there's no use to hoard. But I didn't agree with him and made various reasons, like what if the kid requires some medical treatment, or he doesn't get a job and so on.

What I think that the saying should have some exceptions to be entirely correct.