What can be done in a projectarea, but not with several project areas

Hello,

we are discussing about the organisation of our RTC service, how many project areas do we use and so on.
I know all projects using the same process can be group in on project area, because the process is defined per project area.
I know, that user management will be a point to have a look for, if we are using several project areas.
But what I don't know is the usage-view.
In one project area you can use all items, link them against any other workitems and so on.
I've seen, if using two project areas, I can add dependencies between workitems from both areas.

But what can't be done, using several projectareas?
Or what can be done, but with more effort?

greetings georg.