The biggest issue related to the single site colleciton vs. multiple is the ability to drill down and find information manually. Every companies biggest hurdle is understanding that SharePoint is not just a single web site. It is a complete environment for your company. Also, companies love to adapt new technology, but their mindset on doing business never changes. Meaning that I contain all my customer information in a single manilla folder in a single drawer file cabinet. As I atract more customers I will need to adjust my organizational needs. I don't want to get big new fancy multi-drawer file cabinet and organize my information the same way in the sinlge manilla folder. I would need to re-think my methods, which include identifying the benefits of the new file cabinet and utilizing them to best fit my needs.
So back to SharePoint, with the use of it's many features such as, memberships, my sites, search, and site directorys and the use of well qualified individuals who maintain the content and design of the environment as a whole will go a long way to make life easier in the corporate intranet environment.
Life isn't all about drilling down, users don't go to the internet and drill down through millions of sites to find the one they need, they search. Even in sharepointblogs.com. You won't drill down to find my blog, you will search for it. Better yet, you might save it as a favorite. Bottom line is that the corporate intranet is catching up to the level of the internet, meaning it just isn't a single web site anymore, it is an entire network of sites where each site has it's own uses. This means that there will be multiple personality types using different methods to find the information they need.
As an example... I know that I am not going to find Receipies on how to bake a cake by drilling down through microsoft.com. And if I don't know where it is, then I will have to find another method, which is usually search. Once I find this great place, then I will save it. Its the same for the intranet now. Times are changing quickly and it doesn't matter how big the company is or how many users there are in the organization anymore. What matters is how they do business and how quickly they can react to changes in the business.