passive and active voice

As I teach here in Taiwan, many times I'm told to teach the passive voice to students, and my roommate and I disagree on whether this is correct or not.

EXAMPLE: I ate an apple (active)

The apple was eaten by me (passive)

She argues that passive voice is wrong. I argue that this is something perhaps special about the English language and some others. The fact that - English allows for this flexibility and linguistic mobility. (It's a little crazy sometimes I'll admit - after I teach these kinds of lessons, I find myself turning EVERYTHING into passive/active in my head! I will call my mom...my mom will be callled by me...lol)

Anyway. If it is wrong, why do they even teach it to us in the first place?

My roommate's answer is, they TEACH passive voice or construction, to make us understand that it is wrong and that we should never use it.

But another example; such as...

The girl took some medicine.
Some medicine was taken by the girl.

I don't see anything WRONG about either...grammatically, aren't they correct?

Please try and clear up this confusion if you can - thanks guys!! =)