Why is it that vocal teachers seldom let their students practice singing acapella? I am a person who learn very much from practicing acapella. Why do they seem to dislike acapella and always play the piano?
The only advantage of singing unaccompanied is that it allows you to be inaccurate in pitch and rhythm. A musical accompaniment still allows you to be 'expressive'. But it's the teacher's responsibility to train you in the difference between flexibility and just plain wrong!
It's good to be able to sing a capella without losing pitch and/or floundering rhythmically. But you learn to do this BY singing with accompaniment. And, in most musical circumstances, singing a capella is not the basic requirement.
Singing against piano is very good for getting tuning under control. Few people, and especially not untrained musicians, can sing a tune or a long phrase to the end without drifting in pitch. Also sometimes you think you are in tune, but you are not (it's very easy to be deceived with singing, where what you hear is a bit different to what others hear).