This really does pose the question of what is “built-in material” and what should be delegated to block libraries (with the “out-of-band” possibilities they offer).
To me it deals with both subject domain and complexity of the subject matter.
For instance, I wouldn’t support blocks that do administrative operations - like alter items or things - in the built-in library, because I believe rules in essence are for acting on items, or eventually things (through things actions), but not modifying the configuration.
But if you do want to modify the configuration with Blockly scripts, then you can, but you should install this “admin” block library which will allow you to do that.
On the other hand, while date & time manipulation is desired in rules in general, it is such a complex subject matter that we could perhaps have “go-to” block libraries in the marketplace (the brilliant Date and Time is proof that we can solve such problems with block libraries) for those subject matters. It comes from the observation that we would struggle to get it exhaustively “right” no matter the effort, and libraries would allow a more organic progress.