Google is a corporation. Everything they do is to make money.
The images they're caching aren't mine, anyway, and in many cases they're unsolicited. Sure there's the evil aspect to this (they own advertising), but there is the potential good of obfuscating your actually private data - the IP you check your mail from, when you check it, anything you send back with an HTTP request - from marketers. On solely that note, I'm all for it. But I'm also one of those who like the new Tabs setup, and rarely loaded images for emails from people I don't know.
But in many cases they are solicited, and people want the behavior that gets triggered by knowing about opens:
You always opens our offer e-mails? We'll send you more of the same of what you open, and less of what you don't, increasing the chance you'll find something you like.
Stopping web-bugs from the spammers will improved things, but stopping it from legitimate opt-in marketing mails will make the experience worse and less targeted for people.
The company I work for send millions of e-mails on behalf of customers. All opt-in, and I spend far more time than I'd like making sure we comply with all expectations of the mail providers and ISPs...
But I'm all for Google proxying and hiding IP, cookies etc - I wrote a webmail solution back in the day, and co-founded a company to run it, and frankly I pretty much assumed Google did this already; we did that back in '99 because it was the obvious thing to do.
The images they're caching aren't mine, anyway, and in many cases they're unsolicited. Sure there's the evil aspect to this (they own advertising), but there is the potential good of obfuscating your actually private data - the IP you check your mail from, when you check it, anything you send back with an HTTP request - from marketers. On solely that note, I'm all for it. But I'm also one of those who like the new Tabs setup, and rarely loaded images for emails from people I don't know.