I tested this using P98.
A *very* brief investigation shows that the dominant calendar seems to be the resource pool calendar -- as soon as you assign a resource from the pool to a project, the resource pool's calendar overrides the project calendar.
To me, this is eminently sensible. Instead of having to track stat holidays across multiple calendars, you simply go to the resource pool file. In that file, you carry one holiday calendar for each "group". I have one calendar for US resources, another for UK resources and another for Brazilian resources (CalUS, CalUK, CalBra). Each of those calendars reflects local stat hols.
In my case, because the entire project is in a single file, my project file contains the CalUS, CalUK and CalBra calendars. If I were running a resource pool then that resource pool file would contain the CalUS, CalUK and CalBra calendars.
Each resource is assigned to a base calendar (CalUS, CalUK or CalBra depending on the resource's location). In my case, that's directly in the project file; in a resource pool example, that would be in the resource pool file.
In addition, of course, each resource (because of the way Project works) has a personalized calendar (based on the base calendar such as CalUS, CalUK or CalBra) which includes personal vacation time, etc. In my case, those personalized calendars are in my main file; in the case of a resource pool, those personalized calendars would be in the resource pool file.
Implicit in this discussion is the fact that the project schedule files do not have a calendar and simply use the Standard Calendar without any modifications.
This leads us to the following "best practices":
If you using a resource pool then
(i) all base calendars should be in the resource pool file; and
(ii) all project files should use the "Standard calendar" (which, of course, you should *NEVER* modify).
Your original question was "why should project holidays need to be uniform across all projects?". The answer is: project holidays should be put into calendars in the resource pool only. This will let you control holidays in a central file and the changes will be pushed out automatically and uniformly to the related project schedule files as resources are assigned.