Sharing a single primary key sequence across a database?
Acceptable? Sure. Common? No. Beneficial? Doubtful.

At my old job we inherited a system where they had a central sequence generator (this was a SQL Server system long before SEQUENCE was introduced in SQL Server 2012). It wasn't really a performance bottleneck and shouldn't be unless you're generating hundreds of thousands of values per second. But it made all of the code a lot more complex than it had to be, for no good reason. The intent of the design was to be sure that if something in the system was assigned an ID value of 12, only one thing in the system could have the ID 12. This seemed quite obtuse to me and I never understood it. If I have a customer with CustomerID = 12, why does that preclude me from having an order with OrderID = 12?

I do see the usefulness of a central sequence generator if you have multiple systems and you are generating IDs for a certain type of entity (say, a customer or an order) from these multiple systems. A central sequence can dole out new values to multiple systems without being a bottleneck (just a single point of failure) and without fear of two systems generating the same ID.