We very rarely use tee work with our hitters. Bregman provides us with a great explanation of why.
It’s easy to create bad habits when the environment allows it. For instance, imagine we played with a mound that was 3 feet tall. The ball would arrive at home plate at a much steeper angle than it already does. In order to match the plane of that pitch, we’d need a more uphill swing path. If hit on consistently, our swing would adjust and attack the ball at a much steeper angle.
When we hit off a tee, the opposite happens. Because the ball isn’t moving, we don’t have to adjust to the slight downhill slope that a normal pitch approaches with. As a result, we can get away with a less than optimal swing path while still producing good ball flight.
The tools we use everyday are going to create habits. The habits they create are dependent upon the environment they create. Make sure you create that environment carefully.