"Useful for those who know what they're doing"
Just about anything is useful for those that know what they are doing.
An interface to alter engine timing while you are driving, useful if you know what you are doing. An interface to override coolant flow in a nuclear reactor, useful if you know what you doing. For that latter, there might not be anybody who REALLY knows what they are doing well enough to fuck with it, but that wouldn't stop people who WRONGLY believe they know what they are doing well enough to do so.
This should not be the bar for adding a capability to a browser that is enabled by default. The bar for adding something to a browser that is enabled by default should "will this enable new classes of malware and make the problem of malicious web pages larger than it is today?" and unless you can answer "no" it should NOT be added, or if they are MUST always be disabled by default and appropriate warnings shown if you try to enable it. Just like stuff like a web interface to bluetooth or GPS should NOT be added, or if they are MUST blah blah blah.
Google just wants to destroy the world.
Biting the hand that feeds IT
