I never set this on my chat clients, and find it very annoying that it's not always done for me. I think this one is just a matter of opinion. Perhaps a setting for it would be nice in the future, but for 1.0, I think they made a good decision.
There's a difference between cutting a feature due to resource constraints, and actively saying that the feature is a bad idea. Unless you're pulling an Apple/Microsoft and telling people they're wrong, while you hurry to do the right thing.
I agree. If I don't want to be interrupted, I close down the client. I don't really see how this is much different to manually setting my status to 'busy'.