While I believe private Ethereum chains are invaluable as sandboxing and development environments, I still think they lack fundamental value that is only offered by the public chain. Consensus is expensive; millions of dollars are paid to miners every day to secure major public blockchains and as such the public Ethereum blockchain will always be more secure than a private build protected by a firewall. One of the most beautiful aspects of public Ethereum is that participants all around the world secure their assets natively rather than having to actively build protection to ring-fence data which could very likely be penetrated by an attacker. Currently, if a nefarious actor wanted to attempt a 51% attack on either Bitcoin or Ethereum, they would incur hundreds of millions in cost, possibly more. Once Ethereum moves to the Casper version of proof of stake, the recovery from this type of attack will be swift and extremely expensive for the attacker.