At my school, we have to practice on both sides. We will do triangles ad nausia, and then rinse and repeat on the other side.
You don't punch with only one hand, you don't kick with only one leg. Given, doing one side well before moving onto the other is important, but it should be an eventual goal.
Not practicing both sides is silly.. if someone puts an out and you are used to doing the otherside, you've missed a perfect opportunity to capitalize on a mistake.
Old thread, I know, but I didn't see this.