[FooBarWidget]

I am from Phusion. No idea who downmodded you, but this happens often on HN.

But what you mentioned is not the same thing. In the examples you linked to, Google Analytics and other cookies are removed, leaving only a single cookie. But that's not what we're after. We want to vary by a specific cookie, without removing the other cookies.

For example, Rails stores the session data in a session cookie. We advocate introducing a user_id cookie in addition to the session cookie. Using your approach, the session data would be removed, which breaks the application. Our approach leaves all original cookie data intact while still varying on a specific cookie.

Furthermore, another idea that we've described in the blog post is to vary based some user property, e.g. the user's permission level, in order to increase the cardinality of cache entries. This is not something you can do with only Varnish: it requires cooperation from the app.