Multiple Repositories can bring a lot of advantages and disadvantages. If your company is planning to use multiple repositories, then you will need to make a decision of how many you want and why you want to separate them out like that.

A con of multiple repositories is that if you have many users that need to have access to multiple repositories, then they need a license assigned to them for each repository they are allowed to log in to.

The main thing that is an advantage with multiple repositories is less overlap of templates and fields. If the organization is large and many managers exist then sometimes someone might change a metadata field not realizing some other department was using it a different way and that can be quite annoying.

A great deal of the advantages of the repositories though comes from it's inherent separateness. A truly knowledgeable administrator can easily set up a repository with the proper access rights to make it very much the same as using multiple repositories. Users would only be allowed to see what they should and would never know that other files existed in the same repository when properly configured. In the Administration Console one can also separate out Volumes that the information is stored on from the server side of things and that would also bring with it some similarities to how separate repositories would behave. 

I hope I have not confused you too much, it's just that their are many things that go into the administration side that allow you to get a lot of what you might think you are getting from multiple repositories, just with the use of 1 repository. Hopefully someone else can provide you more insight into how they use multiple repositories or why they do not.