Jeff, how would you advise a player to improve their time without the use of a metronome? Assuming that they've been playing for years, have decent time already, but really want to hone in on their small timing issues? Or would you say that small timing issues aren't really a problem in the first place? And if that's the case, let's assume that they have horrible timing.
It all boils down to this comment: You can't play what you don't know!" Earlier I asked everybody to list anything, absolutely anything in the world that is best learned in time rather than out of it. Nobody offered one suggestion because there aren't any. Everything, absolutely everything is best learned out of time including music.
How do you learn how to play in time without a click? By counting the subdivisions until they are natural in the pieces that you are practicing. By, internally hearing the rhythms, or the quarter note, which is always, without fail, best learned slowly. Good time playing is easy if you already know how to play. Almost without fail, the guys who have time problems are the guys who haven't learned how to play well.