BRK isn't JUST a sort of index fund. Consider its very profitable 2008 investment in Bank of America and its current $10 billion investment in Occidental preferreds to finance the buyout of Anadarko Petroleum.
Buffett can deliver enormous cash almost instantly... and on a handshake after a day or two of diligence. No one can act faster than BRK. That's one reason I own index funds and BRK.