Private keys are not fungible, if you lose one you're done, game over. By putting them in more places you're making yourself strictly less secure.

Also, with that old and worn proverb about eggs and baskets, if having none of your eggs is equivalent in value to you for having only some but not all (i.e. you must have exactly all of them, or else you fail), then putting them all in one basket is better than putting in two (or more).

> By putting them in more places you're making yourself strictly less secure.

If you're trying to minimize baskets, though, then having two very-secure baskets is much better than having N baskets ever-growing because you can't trust in their sturdiness.

Which is to say, putting a USB with your private key on it in your safe-deposit box at your bank, for example, means being more assured about the fact that you'll have a backup copy, meaning you then feel safe excluding it from local hard-disk backups and synchronized remote backups/cloud storage, and have no reason to have it on any computers you aren't currently using for the sake of having somewhere to import it from. In a sense, you've added one vector of attack, but in practice, you've removed several.