Until you're forced to start publishing quarterlies, or until the employees all unwittingly sell to the same would-be hostile takeover buyer.

Why would anyone else be able to sell shares to hostile buyers except employees. Hell, that clearly screws over employees.

That requires that you have IPOed. And that comes with its own set of complications.

Though there are secondary markets arising for this sort of thing.

It requires the SEC stop prohibiting it.