There's a different starting expectation: Anything that fits in the "instant messaging" batch is often used when people wants immediate interaction.

It annoys me to no end - being old school enough to finding it completely acceptable to send a question to someone over IM, and expect them to get it and deal with it later, while many of my co-workers etc. will start any IM conversation with "are you there?" or "hi" and expect a back and forth to confirm this is "live" before telling me what it's about, and effectively removing the choice of dealing with it right away. That's a substitute for the system telling them the message has or will be read.

With that usage pattern - as a slightly less intrusive substitute for a phone call or face to face encounter - knowing that you're not talking to an empty chair helps.

This change effectively shifts Facebook messages further towards the IM spectrum away from the e-mail/letter spectrum. Many e-mail systems support read-receipts, but most people never use them, exactly because a large part of sending an e-mail is that you acknowledge the recipient may want to deal with the message in their own time, or perhaps not at all.

And that's probably why Facebook wants this change: To increase user engagement by getting users to feel more of an obligation to reply and to reply quickly.