The difference is you can't turn it off.

People have different expectations when using this software. I think I've logged on once in the past 6 months. A heavy user who sends me a message and expects, but does not see, a read receipt may think I'm deliberately giving them the cold shoulder.

I want the freedom to not respond or read in privacy. Let them think it was spam filtered or lost in the shuffle.

Yet another reason Facebook is not for me.