From the point of view of the sender, I find read receipts psychologically calming, oddly enough. I noticed I used to attach a lot more emotion to some facebook messages and compulsively check facebook before read receipts. With read receipts, my mind seems to be satisfied with knowing my message was read and now it's up to the other person to answer when they get a chance, or not; emotionally, I seem to be fine with either outcome.

I realize that most other people have a different feeling about it so I thought I'd mention it as an interesting data point.