IMO sparring in the shape of a letter or other symbol is helpful if you don't know what to do in neutral. It gives you something to do while your mind watches the opponent, and you react from your "home" position before returning to neutral. As you improve, you will realize that these shapes limit yourself to a single form of offense (easy to defend) and a predictable form of defense (easy to attack). You effectively block yourself from adapting and challenging an opponent properly as the state of the match changes over time.

If I watch my spacing and force you to back into a wall without either of us getting hit, you will realize you can't do your full letter or shape and will try to find a way to push me back or move to another zone. So I know you'll either be a bit more offensive (lack defense in panic?) or find a zone to run to safety (and I expect this, so reacting and punishing isn't difficult). This kind of thought process happens very quickly, and you won't notice it unless you consider it before the match or are trained to identify the situation as it comes up. Practice makes perfect.