Because of a lack of end-to-end encryption? I'm still trying to wrap my head around it, but it uses TLS, so I'm not sure of the differences.
However, as I said, this wouldn't be a dealbreaker. I wouldn't like using it as much, but it would still be better than nothing.
Also, it works without a data connection.
This is why FB chat, whatsapp, etc will never gain the traction that these services do -- putting time and effort into making them work is just a fool's errand until they open up the network to third party clients and make the specs an open standard.