It seems to be a bit of a holy grail of Service Oriented Architecture, or in fact for any large organisation, to have a single canonical model/form of all important entities to their business.
After watching a colleague struggle with a 900K WSDL that defines something like that (900K of XML !), I happened to stumble across this interesting and amusing blog post: http://service-architecture.blogspot.com/2009/06/single-canonical-form-only-suicidal.html
If SaaS is anywhere in your future, and it will be unless you are a military secure establishment and even then it might me, then GIVE UP NOW on the idea that you can mandate data standards in applications and create a single great big view that represents everything.
I have watched organisations spend millions even trying to define what the most "basic" entity looks like: A Customer ! Its hard to even agree on the basics !
So what ends up happening is that some sort of a standard is reached, and projects have to pay an expensive "architecture tax" to use these huge models, fail, and then feel guilty for creating their own little models that at least allow them to build their app.
With modern mapping tech, such as this or this, the cost of mapping between models is much much less, perhaps its less then the tax of using huge complex models? (this is directly relevant to the models that rules use: rules *can* use the canonical models, depending on how complex you want them to be, but sometimes it is clearer to use a model tailored for where it is used, and map to it from the external model).