Wow, you're right. It's so under the radar, I don't know a single person who has ever used it, or mobileme before it. The point still stands though, in the sense that Mail.app is written with the assumption of IMAP/Exchange as the primary use case, and so they have to assume local processing unless they implement proprietary protocols/IMAP extensions. I don't think you get credit for starting with a legacy POP/IMAP client, and then say "this was architected for local processing for your privacy", in the sense that, it already existed that way because most Mail clients prior to Gmail had to treat the MTA as a relatively commodity store. (you're claiming credit for doing something that was always that way by default) If they launch a rewrite-from-scratch end-to-end client for a proprietary email service protocol, and offer features on parity with gmail, but all with offline processing, then it's worth talking about.

Apple is in the same position as Microsoft with the PC. Microsoft wanted the PC to be the center of the universe, the center of the home, and all processing to take place on it, because that meant MS Software would be in control. Cloud based processing commodifies your platform, as the Web helped commodify Windows and paved the way for the iPhone and Mac to be viable and usable (since most of the apps people wanted to run were now web apps)

Apple wants the iPhone to be the new central hub of everything, because that drives sales of hardware and drives lock-in to their proprietary walled garden. By coincidence, this aligns with enhanced privacy, but Apple is beholden to Wall Street just like Google is, and if there was a new product which threatened to tank their stock, and they had to compromise "principles", you'd start to see a weaseling away.

When Apple reaches saturation with their primary business, the iPhone, and they need to drive huge growth, they'll diversify into other areas. If they are driven into services, they will inevitably run up again the issue of having to process inherently personal data in the cloud.