I'm a little surprised this does not resonate with anyone else. I've used AI for years and have always felt building and updating a "master symbols library" was unnecessarily tedious.
I'd like to see a "Symbol Library Panel", separate from the current document, that could be updated without leaving the current document. When the current document is closed the "Symbol Library" remains active. I realize I can open any AI file as a "Symbol Library" and set it to "Persistent", but it does not update as I work. I must go to the "Library AI file", add the Symbol, close the "persistent library" and reopen it. Makes no sense. I've looked for a 3rd-party plugin that will do this for me.
I fired up InDesign to make sure my memory was correct. Yes, in IND I can "Create new Library" and drag any element into it. I can close and open documents, updating the Library as I work. Makes SO MUCH sense.