The jump seats (there are three in an A380) are used by the SOs and any check personnel.
As to why the rules work the way they do...who can say. Post 9/11 things changed, and mostly not for the better. Anyone carried in the coughpit must have an ASIC (i.e. be staff), they can't be carried in all countries, and they can't leave. As we have a toilet within the coughpit area, this isn't an issue. In the last 7 years, I can only recall one person riding in it, an he was a company pilot on leave.
As we are unlikely to want anyone who isn't crew within the coughpit over a long period it pretty much means nobody but pilots will be offered the seat.