Hi Vincent,
In most cases, I think the default naming makes little sense in real-world models. UML requires NamedElement to have unique names, in a given Namespace, only if their name is non-null.
Users building a real-world model will always give a Name to their elements when they need one. If the element doesn't need a name, then 'null' is a good default (And more efficient than trying to find the next ID increment for each new element)
However, this default naming strategy is still useful when building test/demo/toy models, where the names are required but don't really matter. For such models, performances don't matter either, since they are usually really small (< 100 elements). So having this default naming strategy as an option makes sense, but it shouldn't be there by default.
If there are occurrences where non-null, default names are required, then they should probably be treated specifically (e.g. with dedicated tooling). But I don't think this should be the default for all Papyrus NamedElements, considering the high cost of this strategy.
Cheers,
Camille