It's a question of control. You should be able to control your fingers in every moment, so if you are not able to play exact to the metronome's beats (while you
want to!) this shows a lack of control.
Many people here will be surprised by such a statement of
me 
I don't like the metronme and I don't like metronomic playing. But while noone would expect, I
can play with the metronome, and I do this from time to time to find out the tempo changes in a piece.
There are 2 different ways to play with metronome:
- you play exactly to the beats and play mathematical values of all note durations
this sounds not very musical and I do that only for the fun of showing how one should
not play

- you play almost exactly to the beats, but play with very free rhythm between the beats
this can sound quite musical, however it will not sound very expressive and personal - just the way, most piano teachers will like it to hear from their students
