Are you a pencil or a fish?
Teaching someone to rotate straight through the pool can be a good thing, but it can also limit you to use all your muscles.
It can be restrictive despite the better streanline it produces.
Most people want to uindulate, bounce in rhythm on buoyancy, rotate shoulders more than hips, bend and unbend through the spine etc to get acces to more core power in their stroke.
One of the most pencil like swimmers is Camille Muffat.
Perfect theoretical technique, but not the optimal way for everyone.