This adds a very minor amount of security, if any, if someone manages to hack into your "hot wallet" server they can just initiate a non-multisig transaction and still steal all your bitcoins in that wallet. You can't give the argument that the RPC API is password protected because the password is stored in plain-text in the config so all someone has to do is first grep for the file. There doesn't appear to be a way to force ALL outgoing transactions to be multisig and even if there was one, would you be able to force one of the addresses to be the "hardened" server? That still wouldn't prevent anyone from just stopping bitcoind, changing the config, then restarting it.

Unless you're using your own custom wallet software there doesn't seem to be any sufficient way to prevent someone from stealing all your money once they have access to your server. Other software, like MySQL has access controls so I can prevent ALTERs, DROPs, DELETEs, etc for all "live" accounts limiting the scope of any attack if they manage to get into the server. Maybe this is beyond the scope of bitcoind, not sure.