One of the challenges in the EDW world is extending the data warehouse. If you are adding new data to the warehouse, you will need likely to change the model or at least update the schema and some other metadata. So what is the advantage for the sponsor of the project to add it to the EDW? The temptation is to add a working data store which is related to the EDW but not totally integrated. Certainly not as elegant but can be implemented more quickly.
I expect as a user of the EDW and benefiting from what it offers, the sponsor likely has some obligation to make the new information available in the EDW. In addition the response time with the integrated data is likely much faster and in the overall easier to maintain. In the longer term, integration allows you solution to evolve with the EDW.
My suggestion is to first implement a working data store solution and get payback. If there are advantages in integrating the data into the EDW, then a separate case can be made. The difficulty of the case is that maintenance is one of the hidden cost of a working data store. However I think those trade-offs need to be presented upfront and then later revisited as we proceed.
Another possibility would be develop a phased approach to the project that include the both a working data store approach, a refinement of the design to better meet the broader EDW requirements and integration. All these steps could be outlined up front and decisions made about the correct approach.
The ideal EDW is one with a completely integrated system that respects all the fundamentals of EDW design. We need to aware of the ideal but recognize the realities of the business.
I prefer the phased approach with a Rapid Results spin.
Put the customer requirements at the front of your awareness. Find out how to get them the data they need, then put it in the right place.
This does not mean break the rules, it just means phase in the deliverables and parallel the Enterprise Architecture agenda.