On AIM, logging in was an active choice and it signaled that you were up for a conversation. With Facebook Messenger, everyone who has the app is always "online." This means that:
a) I get messages from people I don't want to have a conversation with at the moment and it seems rude if I ignore them, because there are read receipts.
b) If I send a message, I don't know if they're going to engage in a real-time conversation or if we're going to play phone tag. There's no way of signaling which I'm looking for.