Anon,
To my knowledge, there is no "new" information contained in the Performance Measurement Baseline than what is contained in the combination of the Scope, Cost and Schedule baselines.
I look at it like this. When we speak of 'project performance" we are addressing all three - scope, cost and schedule in combination with each other, not separately. It may be redundant, but project performance hinges on all three baselines in harmony, so it make sense to address them as a group in some circumstances.
Hope that helps,
Harry