@Grace
I totally agree with your frustration. I also had a case where I arranged a language exchange with someone and they decided not to show up at the agreed upon time. Even though we had reconfirmed just 30 mins prior! It felt like he didn't really want to go through with it and perhaps chickened out at the last minute. An excuse was given (he was out at the time) which I found entirely lame given all the lead up to the actual exchange. He suggested meeting another time and I cut my losses and had no use for wasting any more time.
I understand life happens and my current language partner and I have times when things come up, but we give as much advance notice as possible and make sure to communicate clearly so as not to waste the other persons time.
As for the topic of this discussion, I only find it frustrating when people don't read my profile and/or don't even speak a language I'm learning. It's like they just expect free English conversation with nothing to offer in return. Given this is a language learning site, I'm clearly not here to just converse in English.
As an idea, if you find someone you REALLY want to talk to in whatever language and they are not learning your language, at least offer SOMETHING in return. i.e. I know you are not learning my native language X, but would you be willing to learn some basic greetings, etc in exchange for 30 mins of chatting in English? That's something I might at least entertain if I happen to be in the right mood and/or the person is repectful enough to realize they should offer something to the exchange as well. That's why it's called EXCHANGE.