The difficulty is how to work with the ambiguity without blowing your budget or spending more time & money than necessary. Or worse, not meeting the needs of your audience. ie. No one uses it.
Brian's definition of design:
Design consists of creating things for clients who may not know what they want, until they see what you've done, then they know exactly what they want, but it's not what you did.
So what do you do? Who should or could do the design? True business people and IT people think differently. It's why they are in their field of expertise. In some cases, organizations doing BI have instituted a middle-man to coordinate between business and IT. Unfortunately, these positions are dumped on from both sides and rarely carry the authority to make decisions.
An interior designer for building a house isn't there to facilitate between the contractor and the owner. The designer interepts the owner's wants & needs into the language the contractor will understand; but knows the contraints the contractor is restricted by. If you've even looked for a house, you will have seen someone's self-designed interior. The wall colours seemingly picked randomly. The furniture from Ikea and the local-artistic-funky furniture company. The carpet from the 20's. No vision, just ideas from magazine flipping.
For your BI project, find someone who can see both sides - business vision and the technology constraints. It's not important that they know 'how' it will get built in the end. Let them be creative and innovative with the authority to make design choices.