Picking the S&P500 over a world index because you think it will outperform, has the same problem as picking individual stocks over an index. You can't actually know which will outperform in the future.
Or, where in the world do you need to spend your money?
I live in the UK: if I buy the S&P500 over the FTSE100 (or even more so the 250, the next 250 largest companies which are typically more UK-market-oriented) I'm making a US-weighted bet. But maybe I think I'll move there, and should have that exposure. Or maybe I spend a lot of money all over the world and want a more global exposure overall.
I think at least vast majority index is right for basically everybody, but you do still need to think about which index/indices are most applicable to your situation/intentions.
