As a matter of general principle, I strenuously object to any computer I own sending out information about my behavior without my intent or permission. The only egregious offence of the read receipt is that I don't control it.
I classify any case of a computer that's solely mine doing something I don't want as either a bug or malware.
Email clients block HTTP images for a reason. That reason is so that people can't use them to implement read receipts.
Or Javascript. Or non-free software.