Limitations of Having Multiple Clients with their Own Domains on One Installation

I am trying to work out what would be the best approach for having multiple clients. I know it’s technically possible to have multiple domains and that each can have subsites and that makes maintenance much easier. But it seems like they will always be seeing the other domains in the system and sometimes the sites.

What considerations should be made, and the pros/cons for having all the clients with their own networks on the one wpmu installation or to create their own.

For example I know that CampusPress is all about creating a unique install for each campus. They could have mixed them all together into one install but obviously there are important reasons for them not sharing one install.