We're deploying a multisite installation for our new Intranet at work. Essentially, we have a main intranet site (the default install), then we have a site for each department.
The idea was to be that the primary links menu would stay the same across all sites. As there are 7 department sites, we didn't really want to copy and paste the menu 7 times, then edit it 7 times, whenever something changes in it. So, the best solution would be to share the appropriate menu table.