How do you view the rubato used by Stokowski? There's barely a pianist alive who would use a fraction of that rubato. The kind of freedom he employs in the lyrical 2nd subject from the first movement of the Pathetique really is not something that is learned by practising with a metronome.
Yes, that's a good point.
You would not learn to do that by practicing with a metronome. (If you wanted to do that! I like Starker's Bach cello suites and can't stand Yo-Yo's, for precisely that reason.)
Stokowski does not vary back and forth around a steady pulse, he moves the pulse itself. Of course he does so in a more extreme fashion than many would, but all good conductors do some of this.
In contrast, various traditions of jazz band keep more of a steady beat but lay behind it or push ahead of it. In some genre's one section may play ahead while others play behind.
One common factor is that the individual musician must conform to an external time standard. Whether you play for Stokowski or Buddy Rich, you don't do your own rubato. You follow somebody else's.
Personally I think the skill to follow another person is a refinement of the skill to follow a metronome. Being able to follow a metronome would then be a necessary but not sufficient condition. Certainly musicians do learn to follow without large amounts of metronome practice, but I think it helps.