Although it's remarkable that we can intercept and modify our email and web traffic to achieve these effects, few people do it. The benefits of local proxies can be gotten in other ways that feel less subversive.
It's easier to see why the web services fabric increasingly depends on proxies. Fault tolerance, service-level agreements, access control and business activity monitoring are among the many things enabled by intermediaries that watch (and sometimes transform XML) message flows. When those message flows reach the desktop, intermediaries will flourish there too.
As an experiment, I built a simple web proxy that converts incoming HTML to well-formed XHTML. It's a cute trick that enables more powerful kinds of search and transformation than is possible with the regular-expression-based text patterns that the ad blockers use. But well-formedness, though necessary, isn't sufficient. The data must also be self-describing, as web pages mostly aren't but SOAP messages are.
One of these years, my bank will upgrade to a new system that's built around web services. They'll probably offer a basic "rich internet application" -- for Windows, Java or Flash -- that connects to those services. When the bank announces the upgrade, it will stress the richer user experience and choice of interchangeable clients.
Those will be crucial benefits indeed. What won't be said, because it's harder to explain, is that the system will also have become radically extensible. Suppose I want to trigger an alert when a transfer exceeds some limit or when a duplicate amount appears. Today, if the system doesn't implement these rules, I'm stuck. In a services-oriented environment, though, I needn't depend on either the bank or my client software. If neither delivers the features I want, I'll inject an intermediary that does. Local proxies are geeky curiosities today, but someday we'll wonder how we lived without them.Udell is lead analyst for the InfoWorld Test Center.