I had a professor once tell me, in terms of playing the piano, that he would rather see me use my brain for a metronome, rather than my foot. I understand this, and when I'm just playing or recording I typically will use my brain to keep time, but I'm not sure I understand the harm in tapping my foot. Does anybody have any opinions on this?
I've watched people who tap their foot while playing; I've stood next to people in choirs who do it. I don't remember one person who did not experience problems keeping time that way.
It's an extra movement to co-ordinate, and co-ordination can suffer. When these people reached a more complicated section, their foot-tapping slowed down to accommodate it. And they didn't recognise that that had happened. "But I was tapping my foot to keep time!"
With a "mental tap", or merely feeling the beat, there's no additional movement to do and it's far easier to match the movement you have to make, or to realise that you haven't managed it.