I cannot come up with any reason for using it. Also, why are they reengineering the HTTP proxy? We already have those and they work fine. Even if they've designed a better-performing backend, there's just no reason to not use the proxy interface that's built into virtually every web browser.
But, again, why do I want all of my web traffic going through Google's servers? Virtually everyone that uses a proxy is forced to do so by company policy requiring all of their web traffic to be snooped, not by choice, with the rare exception of, basically, avoiding such a policy.
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Bitt Faulk