It's really not even enough to open source their server software because you would still have to trust that they ran that open source software unmodified on their machines.
Moxie's TextSecure on the other hand does full end-to-end encryption, with no work done by any server. Same with OTR and similar message encryption plugins. It baffles me why people are sticking to the "everything on the server" route for applications that focus on user privacy and security in this day and age, especially when there are a lot of good alternatives that don't.