Single organization with single/multiple domains

I am currently design a 6000 seat Domino organization which has 3 primary regions:Americas, Europe, Pacific. Orginally I started designing the system with one organization and one domain however becuase of politics it appears that my client would like 3 seperate orgainizations and seperate domains.

Before I present the Pros/Cons of having seperate orgainzations and domains, I would like to have your thoughts and opinions. I ahve included my pros/cons of having seperate domains.

Note: If there were 3 organizations, they would be cross certified.

Pros of Multiple Domains

Increased security

Might be user for end users (multiple address book)

Cons

Harder to administrator

Not feeling of single company

Pros of a Single Domain

Easy to Administrator

Cons

One change affects every address book

TIA