When using mail most people don't want to let know when they read a mail. I do for instance because it hurts my privacy. If the notification was delayed so that the sender doesn't have the precise time when I red the mail, it could reduce the problem. Though in other context, like intra-work, beeing able to know when someone opened a mail might be of value for both ends.

Add to the equation the spammers who would be delighted to know if their mail was red.

I think you're right. It's really bazaar why Google hasnt implemented those features into Gmail, but if they were seamlessly integrated into Sparrow, I'd pay.

If gmail starts notifying people I've read their email I'll be switching email services...

At the last full-time gig I had we had a plugin installed on the mail server to strip out external read-receipts

We're doing something along those lines. I'll let you know how it goes if you're curious.