When you hold your coins on coinbase you dont control the private keys so its at their discretion if they want to credit the forked coin to you and if the site was ever hacked you would lose all your coins.  Like others have said the smart play for everyone is to store there coins on a hardware wallett Nano Ledger or Trezor. You control the private keys so you can keep your coins completely safe this way and benefit from all the forked coins. Anyone who tells you different is wrong and doesnt understand how the technology works.