When you have a large company, the company is also split over several administrative sites, in some cases you might have a single administrative group covering several sites though, this allows you to provide them with a single /48 as they are one group they will know how to properly divide that address space up.
Works great, until you realize that for traffic engineering purposes, you really want to announce your Los Angeles site at an exchange near there, and your London site to be announced near there, and you end up wondering whether deaggregating the /48, or getting a second/third /48 would be wiser.. ;)
I believe that a separate /48 per site is better regardless of whether or not the company has contracted with a single ISP for all sites, or not. As far as I am concerned if there is a separate access circuit, then it is a site and it deserves its own /48 assignment/allocation. --Michael Dillon