The identity of BBC Parliment is well suited to its purpose. There's simply no point in bringing in the corporate look. It will definitely not be high up (or even on) the Beeb's list of things to do for quite some time!
I don't think it would matter really. They only average an audience of 2, and that's Tony and Alistair.
Is BBC Parliament actually run by the BBC? I know it's a silly question, but it used to be called the Parliamentary Channel, didn't it? Didn't the BBC just buy it and, presumably, allow the previous owners to carry on running it, hence the lack of the corporate graphics?