Many SDKs make it difficult to implement dynamic namespaces without restarting a server.
This means that most basic server implementations will not be able to implement easily.
Vendors who implement generic aggregating servers need to support dynamic namespaces so adding new namespaces for each device is no big deal.

The general guideline that I think we need to make:
1) Adding NodeIds is cheap;
2) Adding NamespaceUris is expensive and should be done only when absolutely necessary.