So why would this redundant data be here? Performance? I doubt it. The whole "immutability" of Rules and Vocabularies makes sense from a version management perspective until you actually start using them. Imagine the scenario; You have defined your Vocabulary before starting your rules (This is required because you can't reference a Vocabulary from your rules until that Vocabulary has been published - this in itself is predicated on knowing all the terms up front - big assumption!). You define 25+ rules, using multiple references to your vocabulary. Fine. You now publish the rule policy. At this point the world is still in order - version 1.0 of both rules and vocabulary are in a lotus type harmony. But what happens when you want to extend or amend the vocabulary? You create a new version of the Vocabulary (cut & paste!),make your amendments and publish it. Of course NONE of your rules will reference this new vocabulary (1.1) as all links reference 1.0 of the vocabulary. The only way to reference the new vocabulary is to create a new version of your rules (cut & paste!), and edit EACH AND EVERY reference to the vocabulary to repoint to the new version.
This isn't new of course, and tools do exist to deal with it.
The BRC has been criticised for this "cut & paste" mentality, but in fact some of the problem goes back to the design of the rules system itself.