Given that using the "Add -> Add Downtime" functionality allows you to add a downtime for any site that you have a suitable role over, there may be some mileage in allowing multiple sites to be selected in a single downtime.

One use case I can think of is being able to define one downtime for events affecting 1 physical location (i.e. the RAL campus), but affect multiple GOCDB sites (GRIDOPS-{APEL, GOCDB}, RAL-LCG2, the STFC cloud).

It might require schema changes, but then again I can't see any reference in the Downtime entity to a Site (presumably because Downtimes affect Services, not Sites).