The catch is - we're closed source, at least for now. We feel that earning people's trust is about more than just dumping the source code.
Our source tree also has a lot of work in progress on cool features that we would like to be the first client to support.
Once we're ready to come out of beta, we'll re-evaluate this decision.
I am well aware of how well this will go down around here and I preemptively support your calls for my head on a spike.
Ultimately, I cannot believe anyone would use this client until the source is posted on Sourceforge or GitHub. Why would anyone want to trust a closed source client. Earning trust can most easily be accomplished by being open and transparent. Don't give people a reason to disbelieve and they won't.
There is no reason not to leave the source open from the beginning, unless you have something to hide. In fact, if you really want it to take off, then you should welcome additional eyes to assist with issues, features, and bugs.